Expand GitHub contributing info

.github/CONTRIBUTING.md

@@ -1,4 +1,26 @@
-Anymail is maintained by its users. Your contributions are encouraged!
+Anymail is supported and maintained by the people who use it—like you!
+We welcome all contributions: issue reports, bug fixes, documentation improvements,
+new features, ideas and suggestions, and anything else to help improve the package.
 
-Please see [Contributing](https://anymail.readthedocs.io/en/latest/contributing/)
-in the Anymail documentation for more information.
+Before posting **questions** or **issues** in GitHub, please check out
+[*Getting support*][support] and [*Troubleshooting*][troubleshooting]
+in the Anymail docs. Also…
+
+> …when you're reporting a problem or bug, it's *really helpful* to include:
+> * which **ESP** you're using (Mailgun, SendGrid, etc.)
+> * what **versions** of Anymail, Django, and Python you're running
+> * any error messages and exception stack traces
+> * the relevant portions of your code and settings
+> * any [troubleshooting] you've tried
+
+For more info on **pull requests** and the **development** environment,
+please see [*Contributing*][contributing] in the docs. For significant
+new features or breaking changes, it's always a good idea to
+propose the idea in the [discussions] forum before writing
+a lot of code.
+
+
+[contributing]: https://anymail.readthedocs.io/en/stable/contributing/
+[discussions]: https://github.com/anymail/django-anymail/discussions
+[support]: https://anymail.readthedocs.io/en/stable/help/#support
+[troubleshooting]: https://anymail.readthedocs.io/en/stable/help/#troubleshooting