Add a github issue template

And urge people not to create issues for things which aren't
actionable for us as poetry2nix maintainers.

.github/ISSUE_TEMPLATE/issue-template.md

@@ -0,0 +1,35 @@
+---
+name: Issue report
+---
+
+<!--
+Is this an issue in the style of "package "X fails to build"?
+
+Most such issues aren't issues in poetry2nix as such, but rather an incomplete dependency specification.
+We don't have the capacity to act on most such issues, they often require domain knowledge and they generate noise in the issue tracker.
+
+The best way forward for these types of issues is _not_ to create an issue but to try to create an override yourself and submit a pull request.
+
+If you are struggling to create a pull request, come to the Matrix chat and we will happily help you through the process.
+-->
+
+### Describe the issue
+<!-- A clear and concise description of what you expected to happen. -->
+
+### Additional context
+
+<!--
+Note that while these aren't strict requirements it will heavily increase your chances to get an issue fixed if you include more context that makes it possible to recreate the issue.
+-->
+
+- `default.nix`/`shell.nix`/`flake.nix`
+```
+```
+
+- `pyproject.toml`
+```
+```
+
+- `poetry.lock`
+```
+```