Module guide

[byorgey]

There are so many modules now that I often have real trouble finding the right place to make the changes I want to make. It would be nice to have some kind of developer guide on the wiki which explains the organization of the codebase and gives some general hints about what can be found in various modules (though we don't necessarily need to list every single module). The challenge, of course, would be keeping it up to date.

[kostmo]

In addition to documentation, #1043 will surely help with this.