README.org: Mention that `runlisp' can support non-free Lisps.

README.org: Mention that `runlisp' can support non-free Lisps.

The old text stated rather baldly that, if you wanted to use a non-free
Lisp system, then `cl-launch' was your only option.  This was basically
true when it was written, because that version of `runlisp' had support
for the known Lisp systems baked into the code.  (Even then, I suppose
you could have maintained a fork with the additional support.)

But now, supporting new Lisp systems is just a matter of writing the
necessary configuration runes in a text file.  This is clearly possible
and practical.  It's just that I'm not interested in carrying
configuration for non-free Lisps as part of the `runlisp' distribution.