RCBC-120 Add note about supported ruby versions

Change-Id: Ib3f9cb32abf4b02be9636755fa9d1d9410500c01
Reviewed-on: http://review.couchbase.org/32334
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>