Updated instructions.

_pages/software_instructions.md

@@ -108,7 +108,14 @@ excerpt: none
 
 #### Update the changelog
 
-Update the changelog file at `CHANGELOG-${project}.md`
+Update the changelog file at `_software/CHANGELOG-${project}.md`:
+
+- update the front matter with the appripriate data:
+  - `updated` and `date` must be the UTC representation of the timestamp of the
+    git commit corresponding to the last published version
+  - `last_version` must correspond to the git tag used for the last published version
+- move the `<!--last_release_end-->` HTML comment after the last line of the new version
+  changelog
 
 #### Update the table of contents