Refresh README.md post kv_engine merge
Refresh README.md post kv_engine merge

Update the top-level README to better describe the unified kv_engine
repo.

Change-Id: Ie4909323512ad307fad7d4323bc939ec3cfb6c7a
Reviewed-on: http://review.couchbase.org/80326
Tested-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>
Reviewed-by: Daniel Owen <b3fc0e1a51f252986e60be7da121b74d3ac6845f@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine