message: docs: update readme about gitter questions
diff: @@ -17,7 +17,9 @@ to find out more about the origin story and design philosophy of Envoy.
  feature design, etc.).
  * [#lyft-envoy](http://webchat.freenode.net/?channels=lyft-envoy): IRC (synced to Gitter)
  * [Twitter](https://twitter.com/EnvoyProxy/): Follow along on Twitter!
  -* [lyft/envoy](https://gitter.im/lyft/envoy): Gitter (most people use Gitter)
  +* [lyft/envoy](https://gitter.im/lyft/envoy): Gitter (most people use Gitter. NOTE: Response to user
  + questions is best effort on Gitter. For a "guaranteed" response please email envoy-users@ per the
  + guidance in the following linked thread)
  Please see [this](https://groups.google.com/forum/#!topic/envoy-announce/l9zjYsnS3TY) email thread
  for information on email list usage.
  