Topic Social Proof Service (TSPS) delivers highly relevant topics tailored to a user's interests by analyzing topic preferences, such as following or unfollowing, and employing semantic annotations and other machine learning models.

User Signal Service (USS) is a centralized online platform that supplies comprehensive data on user actions and behaviors on Twitter. This service stores information on both explicit signals, such as Favorites, Retweets, and replies, and implicit signals like Tweet clicks, profile visits, and more.

Unified User Action (UUA) is a centralized, real-time stream of user actions on Twitter, consumed by various product, ML, and marketing teams. UUA makes sure all internal teams consume the uniformed user actions data in an accurate and fast way.

Since the first batch of open sourcing, we have added the following components:
- User signal service
- Unified user actions
- Topic social proof service

Update the README to include these.

latest code change including the global thread pool

Closes twitter#452
Closes twitter#505

This is to clean up the BCE adapters and services in UUA since BCE no longer exists.

-fix corner case where dr converter failed when initializing

Closes twitter#550

Add the overview readme about how Twitter uses user signals in candidate retrieval.

Open sourcing Aggregation Framework, a config-driven Summingbird based framework for generating real-time and batch aggregate features to be consumed by ML models.

Representation Manager (RMS) serves as a centralized embedding management system, providing SimClusters or other embeddings as facade of the underlying storage or services.

Representation Scorer (RSX) serves as a centralized scoring system, offering SimClusters or other embedding-based scoring solutions as machine learning features.

Tweetypie is the core Tweet service that handles the reading and writing of Tweet data.

Pushservice is the main recommendation service we use to surface recommendations to our users via notifications. It fetches candidates from various sources, ranks them in order of relevance, and applies filters to determine the best one to send.

- renames pushservice readme.md to README.md
- Minor changes to main README.md