note in docs that gevent 1.0 is supported

Change-Id: Ie9d0e4613e06e9e7ff8f6e290d3576106a7786b7
Reviewed-on: http://review.couchbase.org/31189
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>