doc: maybe document compatibility with specific versions of GNU Screen

[salewski]

There is currently nothing in the README.md or other screen-wrappers documentation that itemizes versions of GNU Screen with which the project is known to work.

The intent of the project is to work equally well with all officially released versions of GNU Screen, to the extent that that is possible. But we do not test against ancient versions, and there are issues that come up from time to time that require implementing compatibility changes for specific versions (e.g., issue #1, issue #9, and issue #14).

In a perfect world, we would have an integration testing framework that would test any given screen-wrappers build (or at least any given screen-wrappers release candidate) again different versions of GNU Screen on different platforms. But just having a note indicating "...is known to work with..." or "...has been tested against version(s) A, B, and C..." would be an improvement.