doc: Add section on getting help with codebase

Apparently we didn't actually say anywhere in these docs
where one should go to ask for help with contributing!
(Though we did link to some Zulip-wide resources that included
links to the right channel.)

Fix that.  And take the opportunity to stress the community norms;
we often see new participants who ask for help that clearly
haven't read them.

Author: gnprice

README.md

@@ -58,6 +58,26 @@ and describing your progress.
 [picking an issue to work on]: https://zulip.readthedocs.io/en/latest/contributing/contributing.html#picking-an-issue-to-work-on
 
 
+<div id="getting-help" />
+
+### Asking questions, getting help
+
+To ask for help with working on this codebase, use the
+[`#mobile-dev-help`][mobile-dev-help] channel on chat.zulip.org.
+Before participating there for the first time,
+be sure to take a minute to read our
+[community norms][norms-getting-help].
+
+For more in-depth advice on how to go beyond the minimum
+represented by our community norms, see
+Zulip's [guide to asking great questions][]
+and the resources linked from there.
+
+[mobile-dev-help]: https://chat.zulip.org/#narrow/stream/516-mobile-dev-help
+[norms-getting-help]: https://zulip.com/development-community/#getting-help
+[guide to asking great questions]: https://zulip.readthedocs.io/en/latest/contributing/asking-great-questions.html
+
+
 ### Submitting a pull request
 
 Follow the Zulip project's guide to your first codebase contribution
@@ -116,6 +136,8 @@ see our [full setup guide](docs/setup.md).
 
 If you're having trouble or seeing errors, take a look through our
 [troubleshooting section](docs/setup.md#troubleshooting).
+If that doesn't resolve the issue, see the section above on
+[how to ask for help](#getting-help).
 
 
 ### Flutter version