First draft of developer docs

[dnephin]

These are some notes I wrote while learning how raft works. These docs form a kind of domain model for raft.

The docs can be viewed in github, here's a preview.

Some of the things I found to be particularly valuable are:

• the sequence diagram in apply.md
• an easily discoverable, and comprehensive definition for things like "Applied", and "Committed"
• a list of the threads, and their responsibilities , which can be mapped back to high level operations