DOC: Add policy for people stating concurrent interest

While we formally do not resevere issues, it happens regularly, in
particular on good first issues, that multiple people communicate
interest to work on the issue (most recently in matplotlib#29647).

I feel the need to give a procedure how to handle this. We don't want
person A to state interest, start coding and meanwhile person B
snatches the topic and provides a PR. And I don't want to manually
sort who should be doing what anymore. With this addition we can simply
 point to our guidelines, and people can have clear expectations.

I therefore propose a courtsy time of 24h for not opening a PR if
somebody has stated interest to do so. - The 24h is somewhat arbitrary,
but long enough so that somebody who is actively working on the issue
should produce a result, and short enough to not block other people.

Author: timhoffm

doc/devel/contribute.rst

@@ -318,15 +318,27 @@ chat room and for community questions reach out at community_.
 Choose an issue
 ===============
 
-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.
+In general, the Matplotlib project does not assign issues. You do not need to
+ask whether you can work on an issue. Nevertheless, we encourage following these
+guidelines to prevent duplicated work:
+
+If there is an existing pull request (PR) for the issue, try to work with the PR
+author by submitting reviews of their code or commenting on the PR rather than
+opening a new PR; duplicate PRs may be closed.  However, if the existing
+PR has stalled, and the original author is unresponsive, feel free to open a new
+PR referencing the old one.
+
+If there is no existing PR and it's not obvious that somebody else is working on
+the topic - either by recent discussion or explicit statement, you can start
+working on it right away.
+
+You may optionally leave a comment "I've started working on this"; please only do
+this if you actually work on it, do not use this as reservation because you would
+like to work on it tomorrow or later. Such a comment is not a formal reservation
+or assignment. It's just a way to prevent duplicate work. If there's no follow-up
+discussion or PR within a reasonable time (typically 2 days), we assume that the
+interest has declined, and anybody is free to pick up the topic.
+
 
 .. _how-to-pull-request: