Update build information

Describe how to do customized builds

Change-Id: Iebdab223c61e1d31c0fa80570264d75dbf4d750a
Reviewed-on: http://review.couchbase.org/37268
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>