Add license file

Hi. i am in the process to package quad-rand in fedora and they would like to have a license file include in the crate source.
https://bugzilla.redhat.com/show_bug.cgi?id=1971208
Here the guideline for license in fedora: https://docs.fedoraproject.org/en-US/packaging-guidelines/LicensingGuidelines/#_license_text