add instructions for updating the CHANGELOG

Author: basnijholt

RELEASE.md

@@ -42,6 +42,15 @@ Make an **annotated, signed** tag for the release. The tag must have the name:
 git tag -s v<version> -m "version <version>"
 ```
 
+### Update the changelog
+Use
+```
+docker run -it --rm -v "$(pwd)":/usr/local/src/your-app ferrarimarco/github-changelog-generator -u python-adaptive -p adaptive -t API_TOKEN_HERE
+```
+and commit the relevant parts using
+```
+git commit -p CHANGELOG.md
+```
 
 ### Build a source tarball and wheels and test it
 
@@ -84,7 +93,6 @@ twine upload dist/*
 ```
 
 
-
 ## Update the [conda-forge recipe](https://github.com/conda-forge/adaptive-feedstock)
 
 * Fork the [feedstock repo](https://github.com/conda-forge/adaptive-feedstock)