Mention implementation compatibility in README

[fosskers]

For instance, it reduces maintenance burden if you know which implementations you explicitly support and which are "left to their own devices". re: #120 and my being unable to even build GCL 🤷‍♂️

[kilianmh]

Good idea! We will add it after your PR is merged.

Mainly we develop on sbcl, ccl. Usually we do not test on many others. However so far the tests were succeeding on all tried implementations.

The change #120 is special: if we use a character that is not supported on an implementation (e.g. abcl), there will be an error at load time. Last year #87 had similiar character additions and that made any library depending on str unloadable (e.g. on ccl, lispworks).

[fosskers]

I see this repo doesn't have its tests ran in CI. Has anyone made a docker image / Github Action that has the main implementations installed for quick testing via a matrix? I can't imagine fukamachi at least hasn't thought of this.