GPLv3 short description

Fix the second phrase on GPL on the homepage.

This PR proposes to replace GPLv3 phrase with an enumeration of differences to v2. Simple enumeration is more objective, accurate, and informative.
Any one of its items is or is not of potential interest to licensors. Patents helps with #48 but it doesn't solve it entirely; license compatibility is already of interest in filed issues; 'tivoization' is known to be reason to choose and reason to strongly reject GPLv3. For who doesn't know the latter, search engines are lucky with an unmistakable term.
It has the same wordcount with the text it replaces. :smile: