.github: add CONTRIBUTING.md

.github/CONTRIBUTING.md

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+## Issues
+
+I want to hear about any issues you encounter while using age.
+
+Particularly appreciated are well researched, complete [issues](https://github.com/FiloSottile/age/issues/new/choose) with lots of context, **focusing on the intended outcome and/or use case**. Issues don't have to be just about bugs: if something was hard to figure out or unexpected, please file a **[UX report](https://github.com/FiloSottile/age/discussions/new?category=UX-reports)**! ✨
+
+Not all issue reports might lead to a change, so please don't be offended if yours doesn't, but they are precious datapoints to understand how age could work better in aggregate.
+
+## Pull requests
+
+age is a little unusual in how it is maintained. I like to keep the code style consistent and complexity to a minimum, and going through many iterations of code review is a significant toil on both contributors and maintainers. age is also small enough that such a time investment is unlikely to pay off over ongoing contributions.
+
+Therefore, **be prepared for your change to get reimplemented rather than merged**, and please don't be offended if that happens. PRs are still appreciated as a way to clarify the intended behavior, but are not at all required: prefer focusing on providing detailed context in an issue report instead.
+
+<!-- ## Feature requests
+
+age is small, simple, and config-free by design. A lot of effort is put into resisting scope creep and enabling use cases by integrating and interoperating well with other projects rather than by adding features.
+
+In particular, I'm unlikely to merge into the main repo anything I don't use myself, as I would not be the best person to maintain it. However, I'm always happy to discuss, learn about, and link to any age-related project! -->
+
+## Other ways to contribute
+
+age itself is not community maintained, but its ecosystem very much is, and that's where a lot of the strength of age is! Here are some ideas for ways to contribute to age and its ecosystem, besides contributing to this repository.
+
+* **Write an article about how to use age for a certain community or use case.** The number one reason people don't use age is because they haven't heard about it and existsing tutorials present more complex alternatives.
+* Integrate age into existing projects that might use it, for example replacing legacy alternatives.
+* Build and maintain an [age plugin](https://c2sp.org/age-plugin) for a KMS or platform.
+* Watch the [discussions](https://github.com/FiloSottile/age/discussions) and help other users.
+* Provide bindings in a langauge or framework that doesn't support age well.
+* Package age for an ecosystem that doesn't have packages yet.
+
+If you build or write something related to age, [let me know](https://github.com/FiloSottile/age/discussions/new?category=general)! 💖