Added readme.

src/libraries/System.Net.Quic/readme.md

@@ -0,0 +1,45 @@
+# MsQuic
+
+`System.Net.Quic` depends on [MsQuic](https://github.com/microsoft/msquic), Microsoft, cross-platform, native implementation of the [QUIC](https://datatracker.ietf.org/wg/quic/about/) protocol.
+Currently, `System.Net.Quic` depends on [**msquic@cc104e836a5d4a5e0d324bc08b42136d2acac997**](https://github.com/microsoft/msquic/commit/cc104e836a5d4a5e0d324bc08b42136d2acac997) revision.
+
+## Usage
+
+### Build MsQuic
+
+[MsQuic build docs](https://github.com/microsoft/msquic/blob/main/docs/BUILD.md)
+
+> **Note**: At the moment, we're using stub_tls option to bypass OpenSSL/SChannel, since work with certificates is not fully figured out.
+
+#### Linux
+Prerequisites:
+- build-essential
+- cmake
+- lttng-ust
+- lttng-tools
+
+`dotnet tool install --global`:
+- microsoft.logging.clog
+- microsoft.logging.clog2text.lttng
+
+
+Run inside the msquic directory (for **Debug** build with logging on):
+```bash
+# build msquic in debug with logging and stub tls
+rm -rf build
+mkdir build
+cmake -B build -DCMAKE_BUILD_TYPE=Debug -DQUIC_ENABLE_LOGGING=on -DQUIC_TLS=stub
+cd build
+cmake --build . --config Debug
+
+# copy msquic into runtime
+yes | cp -rf bin/Debug/libmsquic.* <path-to-runtime>/src/libraries/System.Net.Quic/src/
+```
+
+#### Windows
+Prerequisites:
+- Latest [Windows Insider Builds](https://insider.windows.com/en-us/), Insiders Fast build. This is required for SChannel support for QUIC.
+  - To confirm you have a new enough build, run winver on command line and confirm you version is greater than Version 2004 (OS Build 20145.1000).
+
+TODO
+