Document best way for users to get notifications about new guideline versions

[gene1wood]

Add to the docs and site a recommendation about how an administrator that uses the generator to, down the road, get a notification when a new version of the guidelines are released so they can come back and get an update to their generator config.

[nemchik]

#97 possible solution. If releases are tagged here any user can subscribe to release notifications.

[janbrasna]

There is a changelog describing the actual recommendations version changes buried in src/*/guidelines alongside the JSONs — that I'm adding in #255 to the main readme for better visibility.

In #97 I've tagged all the JSON releases to point to the exact version in history, so they can be referenced now, and will also be publishing new versions via "Releases" as the official channel for folks to follow easily.

I'm contemplating a new versioning scheme, where the recommendations and the configs are versioned together (as the latter depend on the former) — in a way that actually any useful output and utilisation of the generator stems from the underlying guideline version, so maybe we can include releases for both (v5.x for the specs, v5.x.y for generator updates) in one timeline?

I will be moving this to a broader discussion, if that major+minor for specs and patch for generator changes works for us, and mainly for the consumers and community in the first place.