readme updates

[jeffpaul]

Is your feature request related to a problem? Please describe.
It's not obvious the level of support provided for this repo, so let's add some clarification.

Describe the solution you'd like

• add support level section and badge
  - [ ] add release version badge
• add license badge

Describe alternatives you've considered
none.

Additional context
n/a

[jeffpaul]

As there's no tagged and numbered release, I'll skip the release version badge for now.

[jeffpaul]

Also, not 100% certain but this appears to be GPLv2, though would be good to get clarification on that before adding in a GPLv2 license badge.

[ericmann]

@jeffpaul The project is GPLv2 or any later version -> https://github.com/10up/wp_mock/blob/master/composer.json#L4