Reword README

This reflects libcouchbase being a mature and well defined project.

Change-Id: Iad12740db2ef8d586036f59731d5e203ef5f5249
Reviewed-on: http://review.couchbase.org/33920
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>