docs: add step in release/README

Author: jpgrayson

contrib/release/README

@@ -47,6 +47,8 @@ StGit Release Checklist
    - Write a sentence or two description of release.
    - Paste this version's changes from CHANGELOG.md.
    - Upload source archive file (`stgit-X.Y.Z.tar.gz`) from step 4.
+   - Upload prebuilt packages from Actions build for this tag.
+     https://github.com/stacked-git/stgit/actions
 
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