message: readme: add contributing section
diff: @@ -45,6 +45,17 @@ to find out more about the origin story and design philosophy of Envoy.
  Please see [this](https://groups.google.com/forum/#!topic/envoy-announce/l9zjYsnS3TY) email thread
  for information on email list usage.
  +## Contributing
  +
  +Contributing to Envoy is fun and modern C++ is a lot less scary than you might think if you don't
  +have prior experience. To get started:
  +
  +* [Contributing guide](CONTRIBUTING.md)
  +* [Beginner issues](https://github.com/envoyproxy/envoy/issues?q=is%3Aopen+is%3Aissue+label%3Abeginner)
  +* [Build/test quick start using docker](ci#building-and-running-tests-as-a-developer)
  +* [Developer guide](DEVELOPER.md)
  +* Please make sure that you let us know if you are working on an issue so we don't duplicate work!
  +
  ## Community Meeting
  The Envoy team meets every other Tuesday at 9am PT. The public Google calendar is here: https://goo.gl/PkDijT
  @@ -57,5 +68,3 @@ If you've found a vulnerability or a potential vulnerability in Envoy
  please let us know at envoy-security@googlegroups.com. We'll send a
  confirmation email to acknowledge your report, and we'll send an
  additional email when we've identified the issue positively or negatively.
  -
  -
  