CONTRIBUTING: add a section about not emailing maintainers

Setup some expectation for etiquette.

CONTRIBUTING.md

@@ -18,6 +18,9 @@ The project currently uses the general CoreOS email list and IRC channel:
 - Email: [coreos-dev](https://groups.google.com/forum/#!forum/coreos-dev)
 - IRC: #[coreos](irc://irc.freenode.org:6667/#coreos) IRC channel on freenode.org
 
+Please avoid emailing maintainers found in the MAINTAINERS file directly. They
+are very busy and read the mailing lists.
+
 ## Getting Started
 
 - Fork the repository on GitHub