Put into README commands to install cross rubies

Change-Id: I93c12b434d93e397e615ff9d09ed15af6570336f
Reviewed-on: http://review.couchbase.org/33298
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>