Add release procedure to README (#58)

README.md

@@ -44,3 +44,19 @@ docker-compose build
 # Just build one image e.g. notebook
 docker-compose build notebook
 ```
+
+## Releasing
+
+Building and releasing new image versions is done automatically via Travis CI. When new commits are
+pushed to the master branch images are built with the `dev` tag and pushed to Docker Hub.
+
+When a new version of Dask is released a PR should be raised to bump the versions in
+the `Dockerfile`s and then once that has been merged a new tag matching the Dask version
+should be pushed. Travis will then build the images and push them with version tags and update
+`latest` too.
+
+```console
+$ git commit --allow-empty -a -m "bump version to x.x.x"
+$ git tag -a x.x.x -m 'Version x.x.x'
+$ git push upstream master --tags
+```