Add a release checklist

Signed-off-by: Simon McVittie <smcv@collabora.com>

release-checklist.md

@@ -0,0 +1,18 @@
+bubblewrap release checklist
+============================
+
+* Collect release notes
+* Update version number in `configure.ac` **and** `meson.build`
+* Commit the changes
+* `make distcheck`
+* Do any final smoke-testing, e.g. update a package, install and test it
+* `git evtag sign v$VERSION`
+    * Include the release notes in the tag message
+* `git push --atomic origin main v$VERSION`
+* https://github.com/containers/bubblewrap/releases/new
+    * Fill in the new version's tag in the "Tag version" box
+    * Title: `$VERSION`
+    * Copy the release notes into the description
+    * Upload the tarball that you built with `make distcheck`
+    * Get the `sha256sum` of the tarball and append it to the description
+    * `Publish release`