Add text of MIT and Apache v2 licenses.
Add initial CONTRIBUTING.md file which specifies contribution license.
Add COPYRIGHT.txt file which specifies the license and lists some
external components.
The LGPL license stays in its current location to avoid having to
modify the build right now.
This commit merges changes that were developed in the "racket7" repo.
See that repo (which is no longer modified) for a more fine-grained
change history.
The commit includes experimental support for running Racket on Chez
Scheme, but that "CS" variant is not built by default.
