CONTRIBUTING.md: Update and expand the release process documentation.

CONTRIBUTING.md

@@ -64,7 +64,9 @@ comparator:
    `diffoscope/external_tools.py`
 
 
-## Uploading the package
+## Release process
+
+### Uploading to Debian
 
 When uploading diffoscope to the Debian archive, please take extra care
 to make sure the uploaded source package is correct, that is it includes
@@ -75,6 +77,8 @@ See [#834315](https://bugs.debian.org/834315) for an example FTBFS bug
 caused by this. (See [#735377](https://bugs.debian.org/735377#44)
 and followups to learn how this happened and how to prevent it)
 
+### Signed tarballs
+
 Please also release a signed tarball:
 
     $ VERSION=FIXME
@@ -85,16 +89,23 @@ And commit them to our LFS repository at:
 
     https://salsa.debian.org/reproducible-builds/reproducible-lfs
 
-After uploading, please also update the version on PyPI using:
+### PyPI
 
-    $ python3 setup.py sdist upload --sign
+You can update the version on PyPI using:
 
-Once the tracker.debian.org entry appears, consider tweeting the release
-on `#reproducible-builds` with:
+    $ python3 setup.py sdist upload --sign
 
-    %twitter diffoscope $VERSION has been released. Check out the changelog here: $URL
+### Docker image
 
-Finally, update the Docker image using:
+Update the Docker image using:
 
     $ docker build --force-rm --no-cache --pull -t registry.salsa.debian.org/reproducible-builds/diffoscope .
     $ docker push registry.salsa.debian.org/reproducible-builds/diffoscope
+
+### Publicity
+
+To announce the package on the `diffoscope.org` homepage, use the `update`
+script in that repository.
+
+Once live entry appears, consider tweeting the release using the release from
+the `@reprobuilds` Twitter account.