Added a pull request template

.github/pull_request_template.md

@@ -0,0 +1,33 @@
+<!-- Thank you for your contribution! -->
+### Changes
+
+Fixes #. <!-- Provide issue number if exists -->
+
+<!-- Please give a short brief about these changes. -->
+
+### Checklist
+
+If this is a user-facing code change, like a bugfix or a new feature, please ensure that
+the you've fulfilled the following conditions (where applicable):
+
+- [ ] You've added tests (in `tests/`) added which would fail without your patch
+- [ ] You've updated the documentation (in `docs/`, in case of behavior changes or new
+features)
+- [ ] You've added a new changelog entry (in `docs/versionhistory.rst`).
+
+If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
+these instructions.
+
+## Updating the changelog
+
+If there are no entries after the last release, use `**UNRELEASED**` as the version.
+If, say, your patch fixes issue #123, the entry should look like this:
+
+`* Fix big bad boo-boo in task groups (#123
+<https://github.com/agronholm/anyio/issues/123>_; PR by Yourname)`
+
+If there's no issue linked, just link to your pull request instead by updating the
+changelog after you've created the PR.
+
+If possible, use your real name in the changelog entry. If not, use your GitHub
+username.