Document and improve how to cut a new release

[choldgraf]

In #717 we're adding a small amount of text on how to cut a new release, however it could be much-improved. This issue is to keep track of this item!

[betatim]

Maybe instead of adding a lot of documentation we can work towards having it automated so that the instructions can be:

• update change log via a PR
• update version tags (app version and chart version, needs some thinking) in the helm chart
• tag a release on GitHub
• open new section in the change log via a PR

Which is roughly what we have on repo2docker. Keeping track of all the twine steps and stuff is too tedious really.

[choldgraf]

good call, updated the title to reflect taht

[minrk]

The chart itself also has a version in helm-chart/binderhub/Chart.yaml. If the two are to have the same version (they do for now, but this is not the case for jupyterhub, and probably will stop being the case with binderhub as it matures), then we need to make sure to update them together.

[consideRatio]

• Closed in favor of Add RELEASE.md and ensure function of automation for tagged releases #1666 with concrete details to complement with