Added a release checklist

RELEASE-CHECKLIST.md

@@ -0,0 +1,25 @@
+# Releasing new versions of Repa packages
+
+1. Update the versions in Cabal files and update the `CHANGELOG.md` files to describe changes associated with these version;
+2. Commit changes;
+3. Tag the commit and push to GitHub:
+
+    ```bash
+    $ git tag -a "vW.X.Y.Z" && git push origin "vW.X.Y.Z"
+    ```
+
+4. Create source distributions:
+
+    ```bash
+    $ cabal sdist all
+    Wrote tarball sdist to
+    (...)/dist-newstyle/sdist/<pkg>-W.X.Y.Z.tar.gz
+    ```
+
+5. Upload to Hackage:
+
+    ```bash
+    $ cabal upload --publish (...)/dist-newstyle/sdist/*
+    ```
+
+That's it!