Document maintenance workflow

[thesamesam]

We should document how to update the action, like:

• what npm invocations are needed (we may want a Docker container given node seems to move slow/fast)
• need to push to the v1 branch too

[radhermit]

I purposely didn't use a docker container because nodejs-based Github actions were (and probably still are but perhaps to a lesser extent) much faster at startup and more importantly it allows more control and internal handling over external actions like caching.

However, I can understand the reasoning behind wanting to move that way if the remaining maintainers aren't overly familiar with node (and/or javascript) and don't want the additional maintenance burden but doing so could lose some auto-handled functionality that the nodejs approach allows.