Add language about not assigning issues (matplotlib#21276)

* Add language about not assigning issues

This is a combination of @anntzer and @jklymak's language on issues not being assigned, though I put it under "new contributor issues"
* https://gitter.im/matplotlib/matplotlib?at=615aca48fb3dcd4e8808f439
* https://gitter.im/matplotlib/matplotlib?at=615acbf9f2cedf67f96662fd

* linewrap

* linewrap

Author: story645

doc/devel/contributing.rst

@@ -40,6 +40,16 @@ difficulty. ``Difficulty: Easy`` is suited for people with little Python experie
 ``Difficulty: Medium`` and ``Difficulty: Hard`` are not trivial to solve and
 require more thought and programming experience.
 
+In general, the Matplotlib project does not assign issues. Issues are 
+"assigned" or "claimed" by opening a PR; there is no other assignment 
+mechanism. If you have opened such a PR, please comment on the issue thread to 
+avoid duplication of work. Please check if there is an existing PR for the 
+issue you are addressing. If there is, try to work with the author by 
+submitting reviews of their code or commenting on the PR rather than opening 
+a new PR; duplicate PRs are subject to being closed.  However, if the existing 
+PR is an outline, unlikely to work, or stalled, and the original author is 
+unresponsive, feel free to open a new PR referencing the old one. 
+
 .. _submitting-a-bug-report:
 
 Submitting a bug report
@@ -73,7 +83,7 @@ If you are reporting a bug, please do your best to include the following:
       >>> platform.python_version()
       '3.9.2'
 
-We have preloaded the issue creation page with a Markdown template that you can
+We have preloaded the issue creation page with a Markdown form that you can
 use to organize this information.
 
 Thank you for your help in keeping bug reports complete, targeted and descriptive.