[docs] community discussions change (#40)

Replace link to openthread-users Google Group with link to GitHub
OpenThread Discussions, clarify issue reporting links.

.github/ISSUE_TEMPLATE.md

@@ -1,3 +1,3 @@
 GitHub Issues are for bugs and feature requests. To make bugs and feature requests more easy to find and organize, we close issues that are deemed out of scope for GitHub Issues.
 
-The openthread-users Google Group is the recommended place for users to discuss OpenThread and interact directly with the OpenThread community. https://groups.google.com/forum/#!forum/openthread-users
+OpenThread GitHub Discussions is the recommended place for users to discuss OpenThread and interact directly with the OpenThread community. https://github.com/openthread/openthread/discussions

README.md

@@ -59,13 +59,11 @@ Please only use the OpenThread name and marks when accurately referencing this s
 
 # Need help?
 
-There are numerous avenues for OpenThread support:
+OpenThread support is available on GitHub:
 
-- Bugs and feature requests — [submit to the Issue Tracker](https://github.com/openthread/ot-rtos/issues)
-- Stack Overflow — [post questions using the `openthread` tag](http://stackoverflow.com/questions/tagged/openthread)
-- Google Groups — [discussion and announcements at openthread-users](https://groups.google.com/forum/#!forum/openthread-users)
-
-The openthread-users Google Group is the recommended place for users to discuss OpenThread and interact directly with the OpenThread team.
+- OpenThread RTOS bugs and feature requests — [submit to the openthread/ot-rtos Issue Tracker](https://github.com/openthread/ot-rtos/issues)
+- OpenThread bugs and feature requests — [submit to the OpenThread Issue Tracker](https://github.com/openthread/openthread/issues)
+- Community Discussion - [ask questions, share ideas, and engage with other community members](https://github.com/openthread/openthread/discussions)
 
 ## OpenThread