docs: add releasing notes

RELEASING.md

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+# Releasing
+
+This is a checklist to use when releasing a new PySTAC version.
+
+1. Determine the next version. We do not currently have a versioning guide, but <https://github.com/radiantearth/stac-spec/discussions/1184> has some discussion around the topic.
+2. Create a release branch with the name `release/vX.Y.Z`, where `X.Y.Z` is the next version (e.g. `1.7.0`).
+3. Update the `__version__` attribute in `pystac/version.py` with the new version.
+4. Update the CHANGELOG.
+    - Create a new header below `## [Unreleased]` with the new version.
+    - Remove any unused header sections.
+    - Update the links at the bottom of the page for the new header.
+    - Audit the CHANGELOG for correctness and readability.
+5. Audit the changes.
+   Use the CHANGELOG, your favorite diff tool, and the merged Github pull requests to ensure that:
+    - All notable changes are captured in the CHANGELOG.
+    - The type of release is appropriate for the new version number, i.e. if there are breaking changes, the MAJOR version number must be increased.
+    - All deprecated items that were marked for removal in this version are removed.
+6. Craft draft release notes (<https://github.com/stac-utils/pystac/releases/new>).
+   These should be short, readable, and call out any significant changes, especially changes in default behavior or significant new features.
+   These should also include a link back to the Github milestone for this release, if there is one.
+   These should _not_ be a complete listing of changes -- those will be auto-generated later, after the tag is pushed.
+7. Commit your changes, push your branch to Github, and request a review.
+8. Once approved, merge the PR.
+9. Once the PR is merged, create a tag with the version name, e.g. `vX.Y.Z`.
+   Prefer a signed tag, if possible.
+   Push the tag to Github to trigger the PyPI publish.
+10. Use the tag to finish your release notes, and publish those.
+    The "auto generate" feature is your friend, here.
+11. Announced the release in [Gitter](https://matrix.to/#/#SpatioTemporal-Asset-Catalog_python:gitter.im) and on any relevant social media.