README.rdoc: mention Mongrel hard dependency
README.rdoc: mention Mongrel hard dependency

We used to support both Mongrel and Webrick. The latter wasn't scalable
(missing heartbeats, ...) so we dropped support for it.

Signed-off-by: Pierre-Alexandre Meyer <ff019a5748a52b5641624af88a54a2f0e46a9fb5@ning.com>

pierre/galaxy,pierre/galaxy,pierre/galaxy