Add notes on releasing gibo

CONTRIBUTING.md

@@ -0,0 +1,20 @@
+# To release a new version
+
+Update the version number, currently found in [gibo](./gibo) and [com.github.simonwhitaker.Gibo.appdata.xml](misc/com.github.simonwhitaker.Gibo.appdata.xml). Then:
+
+```sh
+export GIBO_VERSION=0.0.1 # replace with the updated version number
+git tag -a -m $GIBO_VERSION $GIBO_VERSION
+git push --tags
+```
+
+A new [Docker build](https://hub.docker.com/repository/docker/simonwhitaker/gibo/builds) will be triggered automatically.
+
+## To update Homebrew
+
+```sh
+export GIBO_VERSION=0.0.1 # replace with the updated version number
+export GIBO_URL=https://github.com/simonwhitaker/gibo/archive/${GIBO_VERSION}.tar.gz
+export GIBO_SHA=$(curl -sSL $GIBO_URL | shasum -a 256 | cut -d' ' -f1)
+brew bump-formula-pr gibo --url $GIBO_URL --sha256 $GIBO_URL
+```