Clarify compatibility

[pablobm]

What versions of Rails does Administrate support? What about other libraries?

There's some detection we can do. For example we can show a warning on boot if using an old version of Rails:

• Establish minimum compatible versions.
• Show warnings.

[nickcharlton]

What do you think about this now?

Do we have a set of known incompatibilities we could look at warning for?

As for Rails support, #1740 bumps the minimum to 5.0 and updates it in the README.

[pablobm]

I don't have a list, no. I'm not sure how to best proceed. Perhaps just see what people have issues with, and see how we go.

[nickcharlton]

I think we can roll with it as stuff comes up, iirc we opened this when we were struggling with Rails/dependency version support a bit and hopefully #1740 solves some of that, at least.