Add contributing guide

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+contact_links:
+  - name: Features, Bug Reports, Questions
+    url: https://github.com/basecamp/once-campfire/discussions/new/choose
+    about: Please use the discussions area to report issues or ask quest

.github/ISSUE_TEMPLATE/preapproved.md

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+---
+name: Pre-Discussed and Approved Topics
+about: |-
+  For topics already discussed and approved in the GitHub Discussions section.
+---
+
+** PLEASE START A DISCUSSION INSTEAD OF OPENING AN ISSUE **
+** For more details see CONTRIBUTING.md **

CONTRIBUTING.md

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+# How to contribute to Campfire
+
+Campfire uses GitHub
+[discussions](https://github.com/basecamp/once-campfire/discussions) to track
+feature requests and questions, rather than [the issue
+tracker](https://github.com/basecamp/once-campfire/issues). If you're considering
+opening an issue or pull request, please open a discussion instead.
+
+Whenever a discussion leads to an actionable and well-understood task, we'll
+move it to the issue tracker where it can be worked on.
+
+This is a little different than how some other projects work, but it makes it
+easier for us to triage and prioritise the work. It also means that the open
+issues all represent agreed-upon tasks that are either being worked on, or are
+ready to be worked on.
+
+This should also make it easier to see what's in progress, and to find
+something to work on if you'd like to do so.
+
+## What this means in practice
+
+### If you'd like to contribute to the code...
+
+1. If you're interested in working on one of the open issues, please do! We are
+   grateful for the help!
+2. You'll want to make sure someone else isn't already working on the same
+   issue. If they are, it will be tagged "in progress" and/or it should be clear
+   from the comments. When in doubt, you can always comment on the issue to ask.
+3. Similarly, if you need any help or guidance on the issue, please comment on
+   the issue as you go, and we'll do our best to help.
+4. When you have something ready for review or collaboration, open a PR.
+
+### If you've found a bug...
+
+1. If you don't have steps to reproduce the problem, or you're not certain it's a
+   bug, open a discussion.
+2. If you have steps to reproduce, open an issue.
+
+### If you have an idea for a feature...
+
+1. Open a discussion.
+
+### If you have a question, or are having trouble with configuration...
+
+1. Open a discussion.
+
+Hopefully this process makes it easier for everyone to be involved. Thanks for
+helping! ❤️
+