docs: minimum plugin standards to be listed on the hub

[pnadolny13]

Related to #1292 and #1302

We should document the minimum criteria that we set for plugins to be listed on the hub. One that was proposed in #1302 is that they all have a license.

Some initial thoughts are:

All Plugins:

• install succeeds
• license in repo

Maintenance Statuses - New SDK:

• beta:

  • TODOs from cookiecutter all completed
  • default SDK tests pass

• active:

  • activity in the last X days/weeks/months

Follow up questions:

• What do we do if they stop meeting the minimum standard after being listed already? Is there a grace period before removing?
• What do we do with all the existing plugins that were added before the standards were set? Should they get an exemption?

[tayloramurphy]

@pnadolny13 I'm actually ok with listing plugins that don't have a license, but we should just warn users about that fact and make an issue on the project.

[tayloramurphy]

From OH:

• Testing framework with SDK (Singer Spec at least)
• Publish to PyPI - pinning to specific release
• Static security analysis

[pnadolny13]

This was mostly completed by https://hub.meltano.com/tap-target-maintenance. We can spin off follow on issues if we need them.