doc/releasing: Add instruction to check the package install guide bef…

…ore releasing

Post-release, binary packages are built and then in the last step, installation
of those packages is tested. To test the process out, shell code is extracted from
the relevant document and applied to individual distributions to verify if Snoopy
installs correctly with it.

Therefore, we need to have a matching set of supported distributions in Actions
configuration and in this guide document.

doc/RELEASING-HOWTO.md

@@ -26,6 +26,8 @@ and work out any kinks that get pointed out.
 The following assumptions are, well, assumed, by the releasing scripts:
 - `master` branch is used for releasing.
 - All the code that is to be released is already merged into the `master` branch.
+- Guide `doc/install-from-repo.md` updated with distribution versions that are
+  actually supported (for automated testing in the last step).
 - The final release-related updates to `ChangeLog` and `README.md` files are already
   present in the local files, **but not yet committed**.
 - The release commit does not yet exist.