Added a release checklist

RELEASE-CHECKLIST.md

@@ -0,0 +1,25 @@
+# Releasing a new version of hakyll
+
+1. Update the version in `hakyll.cabal` and update the `CHANGELOG.md` file to describe changes associated with this version;
+2. Commit changes to `hakyll.cabal` and `CHANGELOG.md`;
+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 a source distribution:
+
+    ```bash
+    $ cabal sdist
+    Wrote tarball sdist to
+    (...)/dist-newstyle/sdist/hakyll-W.X.Y.Z.tar.gz
+    ```
+
+5. Upload to Hackage:
+
+    ```bash
+    $ cabal upload --publish (...)/dist-newstyle/sdist/hakyll-W.X.Y.Z.tar.gz
+    ```
+
+That's it!