add a Help tab on the settings screen

[pbiron]

We should add a Help tab on the settings screen that, among other things explains how the Beta/RC streams work (e.g., that you already have to have updated to a nightly before that option can be selected; and the confusion that might ensure if the trunk has already been branched).

We could probably explain that better in the readme as well.

see https://wordpress.org/support/topic/skips-version-5-4-rc1-and-jumps-to-5-5-alpha/

[afragen]

So a Help tab after the Extra Settings tab or a help tab in the something like a screen options dropdown? I'm assuming an extra tab.

[afragen]

The following is a start. 40c7038