Avoid confusing released and unreleased features in docs

[casey]

The docs in the readme and the docs at https://just.systems reflect the current master branch, not the current release, which is confusing. This came up most recently in #1467.

One possible solution is:

• Only update the docs at just.systems on new releases, not on every change to master
• Note at the beginning of the readme that the docs are for current mster, not the current release, and point users to the docs at just.systems for the current release documentation

[runeimp]

This falls inline with my prior suggestion of putting the version of Just that implement the feature. That way it's easy to realize to check the version of just you using when looking at a new feature to you. Maybe default to v1.0.0 for now all the pre v1.0.0 features. Then you only have to get specific since that point which hopefully isn't too much trouble.