maint: more info about tags in HACKING.

* HACKING: more info about tags.
diff --git a/HACKING b/HACKING
index 1d75528..37d5339 100644
--- a/HACKING
+++ b/HACKING
@@ -630,8 +630,23 @@
     make GNUPLOADFLAGS='--user KEY' git-upload-release
 
 * Push the new release tag: git push origin master tag v$(VERSION)
-  You can view unpushed local-only tags with git push --tags --dry-run
-  (and push them by omitting the --dry-run, if there are no extras).
+  View all tags in the repo: git tag -l
+  View unpushed local-only tags: git push --tags --dry-run
+   (and push them by omitting the --dry-run, if there are no extras).
+  If you forget to make the tag at the time of the release, you can push
+   it at any time. But if you want to make the date be the same as the
+   actual release (from https://stackoverflow.com/questions/4404172):
+    commit=.... # prefix of the release commit we want to tag
+    git checkout $commit
+    GIT_COMMITTER_DATE="$(git show --format=%aD | head -1)"
+    echo $GIT_COMMITTER_DATE # make sure it's as expected
+    # where VERSION is the automake version we want for the tag name,
+    # e.g., 1.18.1
+    git tag -a v$VERSION -m"v$VERSION"
+    git push origin --tags # push
+    # return to normal head:
+    git checkout master
+    unset GIT_COMMITTER_DATE
 
 * For stable releases you also need to update the manuals on www.gnu.org.
   - Check links (except do this before the release):