Added small bit in documentation about ForeignKeyField's related_name
Sick, I'll take the gitignore too!

I had a lengthy argument with a co-worker about the implicit `related_name` backreference created by peewee when declaring a `ForeignKeyField`. He was convinced it didn't exist, since it wasn't in the documentation. Only after walking him through the source code and demonstrating it did he believe me. So, for the future, I added a small section in the `ForeignKeyField` docs explaining. 

I do not know how to build/test `peewee`'s documentation but would be glad to if I had some direction. Also accepting changes to what I wrote, if it doesn't match up with the original intent or design.

Also added a `.gitignore` to ignore `.pyc` files and the `build` directory. We can trim that out of here if you feel it doesn't belong, though.
