Here is a collection of guides that might help you get involved with Matrix.
The (updated) Client-Server API guide explains in detail how to use the CS API, which is useful if you want to write a client (or modify an existing one) - or if you're just interested in how it works "under the hood".
If you were using the old v1 CS API, there is also the v1 migration guide which justs lists the changes from v1 to r0.
Let's Encrypt Matrix explains how to use Let's Encrypt's certificates with your Synapse installation. This guide was written by William A Stevens.
The Application services guide introduces and explains Application services, and what they can be used for.
Types of Bridging should be read by all bridge developers to ensure everyone has the same mental map of terminology when implementing bridges.
The End-to-end Encryption Implementation Guide is intended for client developers who wish to add support for end-to-end encryption.