Add releasing notes

RELEASING.md

@@ -0,0 +1,22 @@
+# Releasing
+
+1. Determine the next version.
+   We follow [semantic versioning](https://semver.org/).
+2. Create a release branch named `release/vX.Y.Z`, where `X.Y.Z` is the next version.
+3. Update [version.py](pystac_client/version.py) with the new version.
+4. Update [CHANGELOG.md](CHANGELOG.md).
+   1. Add a new header under "Unreleased" with the new version and the date, e.g. `## [vX.Y.Z] - YYYY-MM-DD`.
+   2. Audit the changelog section to ensure all changes are captured.
+   3. Add a new link reference at the bottom of the changelog with the new version.
+      Follow the format from the previous version links.
+   4. Update the "Unreleased" link to reference your new version.
+5. (optional) Build the package locally and inspect its contents: `pip install build && python -m build`
+6. Open a pull request for your `release/vX.Y.Z` branch against the appropriate branch (either `main` or a version branch, e.g. `v0.3`).
+   Include a section in the pull request description with the CHANGELOG contents for this version.
+7. After pull request merge, create an annotated tag for your version, e.g. `git tag -a vX.Y.Z`.
+   The contents of the annotated tag should be the contents of the changelog for this version.
+   Make sure to remove any leading `#` characters, as they will be considered comments in the git tag body.
+8. Push the tag.
+   This will trigger [the Github release workflow](.github/workflows/release.yml) and publish to PyPI.
+9. [Create a new release on Github](https://github.com/stac-utils/pystac-client/releases/new) pointing to the new tag.
+   Include the CHANGELOG notes from this version.