Add docs about updating changelog

Author: lossyrob

docs/contributing.rst

@@ -57,3 +57,19 @@ To format code:
    > yapf -ipr tests
 
 You can also run the ``./scripts/test`` script to check flake8 and yapf.
+
+CHANGELOG
+^^^^^^^^^
+
+PySTAC maintains a `changelog  <https://github.com/stac-utils/pystac/blob/develop/CHANGELOG.md>`_
+to track changes between releases. All PRs should make a changelog entry unless
+the change is trivial (e.g. fixing typos) or is entirely invisible to users who may
+be upgrading versions (e.g. an improvement to the CI system).
+
+For changelog entries, please link to the PR of that change. This needs to happen in a few steps:
+
+- Make a PR to PySTAC with your changes
+- Record the link to the PR
+- Push an additional commit to your branch with the changelog entry with the link to the PR.
+
+For more information on changelogs and how to write a good entry, see `keep a changelog <https://keepachangelog.com/en/1.0.0/>`_