add some releasing instructions (#500)

releasing.md

@@ -0,0 +1,11 @@
+# Releasing mjolnir
+
+1. Create a new branch and edit the `version` variable of `package.json` to reflect the new version
+2. Once that branch has been merged, switch back to the main branch and pull in the new changes
+3. Tag the new version, ie `git tag -s vX.Y.Z` (where vX.Y.Z is the new version), and push the tag
+4. Once the tag has been pushed, draft a new release on github: https://github.com/matrix-org/mjolnir/releases/new,
+using the Generate release notes button to automatically create the release notes/changelog
+5. Double-check that everything is correct and make any changes as necessary, then publish the release
+6. Publishing the release should kick off a Github Action to build and push the release to Dockerhub -
+verify that this did occur successfully
+