Add bug triaging guide

Implements [Misc #15943]

doc/bug_triaging.rdoc

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+= Bug Triaging Guide
+
+This guide discusses recommendations for triaging bugs in Ruby's bug tracker.
+
+== Bugs with Reproducible Examples
+
+These are the best bug reports.  First, consider whether the bug reported is
+actually an issue or if it is expected Ruby behavior.  If it is expected Ruby
+behavior, update the issue with why the behavior is expected, and set the
+status to Rejected.
+
+If the bug reported appears to be an actual bug, try reproducing the bug with
+the master branch.  If you are not able to reproduce the issue on the master
+branch, try reproducing it on the latest version for the branch the bug was
+reported on.  If you cannot reproduce the issue in either case, update
+the issue stating you cannot reproduce the issue, ask the reporter if they
+can reproduce the issue with either the master branch or a later release,
+and set the status to Feedback.
+
+If you can reproduce the example with the master branch, try to figure out
+what is causing the issue.  If you feel comfortable, try working on a
+patch for the issue, update the issue, and attach the patch.  Try to figure
+out which committer should be assigned to the issue, and set them as the
+assignee, and set the status to Assigned.
+
+If you cannot reproduce the example with the master branch, but can reproduce
+the issue on the latest version for the branch, then it is likely the bug has
+already been fixed, but it has not been backported yet.  Try to determine
+which commit fixed it, and update the issue noting that the issue has been
+fixed but not yet backported.  If the ruby version is in the security
+maintenance phase or no longer supported, change the status to Closed. This
+change be made without adding a note to avoid spamming the mailing list.
+
+For issues that may require backwards incompatible changes or may benefit from
+general committer attention or discussion, considering adding them as agenda
+items for the next committer meeting (https://bugs.ruby-lang.org/issues/14770).
+
+== Crash Bugs Without Reproducers
+
+Many bugs reported have little more than a crash report, often with no way to
+reproduce the issue.  These bugs are difficult to triage as they often do not
+contain enough information.
+
+
+For these bugs, if the ruby version is the master branch or is the latest
+release for the branch and the branch is in normal maintenance phase, look at
+the backtrace and see if you can determine what could be causing the issue.
+If you can guess would could be causing the issue, see if you can put together
+a reproducible example (this is in general quite difficult).  If you cannot
+guess what could be causing the issue, or cannot put together a reproducible
+example yourself, please ask the reporter to provide a reproducible example,
+and change the status to Feedback.
+
+if the ruby version is no longer current (e.g. 2.5.0 when the latest version
+on the ruby 2.5 branch is 2.5.5), add a note to the issue asking the reporter
+to try the latest ruby version for the branch and report back, and change the
+status to Feedback.  If the ruby version is in the security maintenance phase
+or no longer supported, change the status to Closed. This change can be made
+without adding a note.
+
+== Crash Bugs With 3rd Party C Extensions
+
+If the crash happens inside a 3rd party C extension, try to figure out which
+C extension it happens inside, and add a note to the issue to report the
+issue to that C extension, and set the status to Third Party's Issue.
+
+== Non-Bug reports
+
+Any issues in the bug tracker that are not reports of problems should have
+the tracker changed from Bug to either Feature (new features or performance
+improvements) or Misc. This change can be made without adding a note.
+
+== Stale Issues
+
+There are many issues that are stale, with no updates in months or even years.
+For stale issues in Feedback state, where the feedback has not been received,
+you can change the status to Closed without adding a note.  For stale issues
+in Assigned state, you can reach out the assignee and see if they can update
+the issue.  If the assignee is no longer an active committer, remove them
+as the assignee and change the status to Open.