Docs: Contributing

[akaczano]

This section should contain all the information needed to get started working on Burroughs. I'll leave the details of how that is done unspecified, but I am imagining something like:

• A page on how to get setup, where all of the code is located, and how to run the test cases.
• A page or two on the core Burroughs code, focusing primarily the Burroughs, QueryExecutor, and SimpleQuery classes.
• A page on the Single Message Transforms
• A page on user-defined KsqlDB functions (if we have any)
• A page on the front-end code