Update README for latest changes

- Declare dependencies
- Provide note about OS X OpenSSL
- Add mention of release23 branch
- Remove mention of packet-ng branch
- Remove reference to 1.x blog - we're moving away from that model
Change-Id: I8399895bf740df114b082dba33c2d0c8c96ba9f8
Reviewed-on: http://review.couchbase.org/38595
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>