Update CONTRIBUTING and README docs

Author: caitmuenster

CONTRIBUTING.md

@@ -71,5 +71,4 @@ If you're not sure about that, just ask.
 
 ## Getting help
 
-If you need help with writing a WebExtension, email the [dev-addons mailing list](https://mail.mozilla.org/listinfo/dev-addons) or contact the
-WebExtensions team in the #webextensions IRC channel on irc.mozilla.org.
+If you need help with writing a WebExtension, please post on our [add-ons community forum](https://discourse.mozilla.org/c/add-ons/development/) or ask in the [Add-ons channel](https://chat.mozilla.org/#/room/#addons:mozilla.org) on [Mozilla's Matrix instance](https://wiki.mozilla.org/Matrix). 

README.md

@@ -2,7 +2,7 @@
 
 [https://github.com/mdn/webextensions-examples](https://github.com/mdn/webextensions-examples)
 
-Maintained by the [MDN team at Mozilla](https://wiki.mozilla.org/MDN).
+Maintained by Mozilla's [Add-ons team](https://wiki.mozilla.org/Add-ons).
 
 WebExtensions are a way to write browser extensions: that is, programs
 installed inside a web browser that modify the behavior of the browser or