Update contributors guide with conventions on issue assignment

I'm not sure what the best way is to mark that someone is interested in working on an issue. In JIRA you can just assign an issue to yourself but it does not seem that GH issues has the same capability (you have to have write permission to be able to assign issues I think). So maybe the best we can do is ask people to leave a comment if they start working on an issue?

Or, we can ask people to create an empty draft PR and reference the issue so that the PR is linked to the issue even if it doesn't have any associated work.

Locate the contributors guide and read its current issue-assignment conventions. Resolve whether contributors should comment or open an empty draft PR, then document the agreed workflow and verify that the guide clearly explains how interest is recorded.

Written by the indexing model from the issue text.