golang/xenlight: add necessary module/package documentation

golang/xenlight: add necessary module/package documentation

Add a README and package comment giving a brief overview of the package.
These also help pkg.go.dev generate better documentation.

Also, add a copy of the LGPL (the same license used by libxl) to
tools/golang/xenlight. This is required for the package to be shown
on pkg.go.dev and added to the default module proxy, proxy.golang.org.

Finally, add an entry for the xenlight package to SUPPORT.md.

Signed-off-by: Nick Rosbrook <rosbrookn@ainfosec.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>