Add strong consistency dev docs

This page explains the integration points between the Riak KV store and
the riak_ensemble library used to implement strongly consistent
operations.  The library comes with its own developer documents, which
are referenced here.

The script docs/update_line_numbers.erl can be used to keep references
to functions in the docs in sync with the code. Just run it like this
after compiling riak_kv:

escript docs/update_line_numbers.erl ebin docs/*.md