Add developer documentation

This readme file explains riak_ensemble to developers wishing to
understand what the library does and how it is implemented in the code.
It is a guide for developers who wish to contribute or maintain it, with
hopefully plenty of explanations and diagrams to help them quickly grasp
all the information.

It also includes a script to help maintain references to functions in
the code.  Run the update-doc-lines make target after compiling the
library to update the markdown, then check in the changes.
Fix one function reference
Improve line updating script
Address review comments
Merge pull request #53 from basho/feature/add-docs

Add developer documentation

Reviewed-by: engelsanchez