readme.md in each source directory to describe what it does

[m-mcgowan]

Makes it easier for newcomers to know where to put source code and where to look to find existing sources.

Also an overview of the main folder structure in the root readme.md would be useful.

[a2hill]

This would be very helpful. I'm interested in helping or poking around at the very least, but honestly I'm not too sure where to start. The main project readme isn't too helpful either