ENH: repo stat badge

[aburrell]

Requested feature

We can add a badge that shows the current development status of this project; will likely be required for pyHC at some point

The problem or gap this feature will address

Will let people know how active we are at the moment

The desired solution

Here's the link: https://www.repostatus.org/

Possible alternatives

There may be other places that do these badges?