Clarify license in LICENSE.md.
Clarify license in LICENSE.md.

sagemathinc/smc,sagemathinc/smc,tscholl2/smc,DrXyzzy/smc,tscholl2/smc,sagemathinc/smc,DrXyzzy/smc,tscholl2/smc,tscholl2/smc,DrXyzzy/smc,DrXyzzy/smc,sagemathinc/smc,tscholl2/smc