Beef up README

- Remove mention of libcouchbase.pxd
- Provide 'SUPPORT' section
- Specify specific libcouchbase version requirements
- Change link from libcouchbase github repo to official couchbase page

Change-Id: Id649d08f38771b6cb9038d6be2b920221adcbc06
Reviewed-on: http://review.couchbase.org/26385
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Matt Ingenthron <matt@couchbase.com>