Update RELEASE.md checklist w/ command to set NEXTVERSION

RELEASE.md

@@ -2,6 +2,17 @@
   heeding the Versioning Strategy (see
   https://ncas-cms.github.io/cf-python/releases.html#versioning-strategy).
 
+* Set the `NEXTVERSION` version marker across the codebase (added in PRs
+  to mark the next version where the exact number/name is not yet decided)
+  by recursively finding all occurences within the `cf` directory and replacing
+  them with the upcoming version name `X.Y.Z` (replace `X`, `Y` and `Z` with
+  appropriate numbers), via running this command in `cf-python` repo root
+  directory (don't run it repo-wide or it will e.g. edit this script!):
+
+  ```console
+  $ find cf/ -type f | xargs sed -i 's/NEXTVERSION/X.Y.Z/g'
+  ```
+
 * Change the version and date in `cf/__init__.py` (`__version__` and
   `__date__` variables)