Add release instructions to DEVELOPER.md

Author: sgraf812

DEVELOPER.md

@@ -9,3 +9,13 @@ CI on GitHub Actions
 The GHC workflow file `.github/workflows/haskell-ci.yml` is generated by:
 
     haskell-ci regenerate
+
+Releasing a new version of happy
+================================
+
+There are release scripts `./hackage-upload.sh` and `lib/hackage-upload.sh` to upload new releases to hackage.
+These scripts include some sanity checks for issues that have caused problems for past releases.
+`lib/hackage-upload.sh` must be run before `./hackage-upload.sh`, otherwise the latter fails.
+Both scripts take an optional `--publish` argument that is forwarded to `cabal upload`.
+If this flag is not present, only a package candidate will be uploaded.
+