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
Address review comments

Using n_val of 3 and 5 and actual ensemble names as examples.
Merge pull request #1018 from basho/feature/add-sc-docs

Add strong consistency riak_kv integration dev docs

Reviewed-by: engelsanchez