Add source documentation

[TheBeardedQuack]

Adding documentation to explain how the implementation works will benefit future development by enabling new contributors to clearly how the mod is made. The documentation can also provide a reference for later use, I assume everyone knows the typical benefits of clearly commenting code so I won't go over everything.

I've started working on this myself on a fork. Contributions welcome, merge is welcome if you're happy with the work. I will also submit a pull request when I'm happy with the coverage.