Short description of GPL on homepage

Small update of the language in GPL short presentation to convey the copyleft requirement some better. I hope it's better... Also it follows @Lennie's suggestion on downstream recipients ("to whomever you distribute to"), although I'm not sure that particular part, while correct, is essential to the license.

The short description should IMHO cover the essentials, one needs to know, it's way too short for specific details... This is probably not yet ideal. Thanks for taking a look though.