[Internal] Documentation: Adds msdata/direct Sync-up Guide.

docs/sync_up_msdata_direct.md

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+# Update msdata/direct Repo with Cosmos v3 and Direct Codebase
+
+## Table of Contents
+
+* [Background.](#background)
+* [Prerequisites.](#prerequisites)
+* [Steps Required to Update msdata direct Repo.](#steps-required-to-update-msdata-direct-repo)
+* [Validating the sync-up.](#validating-the-sync-up)
+* [Submit Pull Request to msdata direct.](#submit-pull-request-to-msdata-direct)
+* [Sample Pull Requests to Sync-up msdata direct.](#sample-pull-requests-to-sync-up-msdata-direct)
+
+## Background
+
+As a developer on the Cosmos SDK team, we often engage in a task, that requires code changes in both cosmos dotnet sdk v3 repository, as well as in the `msdata` cosmosdb direct codebase (aka `Microsoft.Azure.Cosmos.Direct` namespace). Therefore, sometimes it's utterly challenging to visualize the code changes as a whole, and analyze the impacts. To overcome this, we have created a branch called `msdata/direct` within our cosmos dotnet sdk v3 codebase, that basically mimics the code present in `msdata` repository mentioned above. This simplifies the code changes required to be done in both, provides much better understanding on the overall impacts of the code changes and gives transparency to users in terms of what is the source code for the `Microsoft.Azure.Cosmos.Direct` package.
+
+## Prerequisites
+
+Before covering the sync-up process in detail, please follow the below steps to make sure all the required pre-requisites are met.
+
+### Clone the Azure Cosmos DB .NET SDK Version 3 Repo
+
+- Clone the azure `cosmos-db dotnet sdk` repo in the local environment, using the below git command:
+    -       git clone https://github.com/Azure/azure-cosmos-dotnet-v3.git
+
+- Navigate to the directory `azure-cosmos-dotnet-v3` and check out the following branch, `msdata/direct` using the below git commands:
+    -       git pull && git checkout msdata/direct
+
+### Clone the `CosmosDB` Repo hosted in msdata
+
+- Clone the CosmosDB repository in the local environment, using the CosmosDB onboarding guide. Please note, building the entire repository is not required for the sync-up process.
+
+- Navigate to the cloned `CosmosDB` directory and check out the following branch, `master` using the below git commands:
+    -       git pull && git checkout master
+
+## Steps Required to Update msdata direct Repo
+
+### Create a Feature Branch for Local Changes.
+
+The first step to sync up the `msdata/direct` repo is to create a feature branch out of it, where all the required changes could be made. Later on, we will use the feature branch to submit pull request to `msdata/direct`. Please use the following git command to create the feature branch:
+
+- Stay on the `msdata/direct` branch and run `git checkout -b users/<user_name>/update_msdata_direct_<mm_dd_yyyy>` to create the feature branch.
+
+### Merging the cosmos db v3 Code into Feature Branch.
+
+The next step is to port the latest `master` branch code into the newly created feature branch. Please see the below git commands to perform this action:
+
+- Make sure the `master` branch is up-to-date.
+- Stay on the newly created feature branch `users/<user_name>/update_msdata_direct_<mm_dd_yyyy>` and run `git merge master`.
+- There are likely to be conflicts during the merge. If that happens, we will need to resolve the conflicts gracefully by accepting the incoming `master` branch changes.
+
+### Pick the Required Microsoft Azure Cosmos.Direct files into `msdata/direct` repo.
+
+This is the last part for the sync-up process. Please follow the below steps to copy the required `Microsoft.Cosmos.Direct` files from msdata CosmosDB repo.
+
+- Open command prompt/windows terminal and navigate to the following directory `Microsoft.Azure.Cosmos\src\direct` inside the cloned cosmos v3 repo.
+- Locate and edit the following line in the `msdata_sync.ps1` script with the respective location of the msdata repo: `$baseDir    = "<src_directory>\CosmosDB"`
+- Run the powershell script using: `.\msdata_sync.ps1`. You will notice the script started copying the required files from the msdata repo, and generating the console logs, like the below:
+
+    ```
+    Copying Files: rntbd2
+    Copying Files: AccessCondition.cs
+    Copying Files: AccessConditionType.cs
+    Copying Files: Address.cs
+    Copying Files: AddressCacheToken.cs
+    Copying Files: AddressEnumerator.cs
+    Copying Files: AddressInformation.cs
+    Copying Files: AddressSelector.cs
+    Copying Files: ApiType.cs
+    Copying Files: Attachment.cs
+    Copying Files: AuthorizationTokenType.cs
+    Copying Files: BackoffRetryUtility.cs
+    Copying Files: BadRequestException.cs
+    Copying Files: BarrierRequestHelper.cs
+    ```
+
+- Note: There may be instances where some of the files could be missing in the v3 `msdata/direct` repo and the copy may fail with the following error: `Write-Error: SystemSynchronizationScope.cs False`. If that happens, please copy the file manually from the `msdata/CosmosDB` repo and continue running the script all over again. 
+
+## Validating the sync-up
+
+One of the most important part in the whole `msdata/direct` sync up process is to validate whether the code merges, conflict resolutions and file updates went successfully. To comply with this, please make sure to follow the below steps:
+
+- Open command prompt/ windows terminal and navigate to the directory where the cosmos v3 code is located, for instance `C:\stash\azure-cosmos-dotnet-v3`.
+- Make sure to stay on the newly created feature branch.
+- Stay on the same directory mentioned above, and run the following command for a clean build: `dotnet build`. Make sure, the build passes successfully.
+
+## Submit Pull Request to msdata direct
+
+Once the feature branch builds successfully, it's time to submit the PR to `msdata/direct` to complete the sync-up process. To do this, please follow `git add`, `git commit` and `git push` commands to push the newly created branch upstream. Once the branch is pushed, please submit the pull request to the `msdata/direct` branch and seek for approvals.
+
+## Sample Pull Requests to Sync-up msdata direct
+
+- [[Internal] Msdata/Direct: Refactors msdata branch with latest v3 and direct release](https://github.com/Azure/azure-cosmos-dotnet-v3/pull/3726)
+- [[Internal] Msdata/Direct: Refactors msdata/direct branch with latest v3 master and Cosmos.Direct v3.30.4](https://github.com/Azure/azure-cosmos-dotnet-v3/pull/3776)