Update README, CONTRIBUTING and LICENSE.
This commit
- updates the Readme to match qmlweb2. It's not yet finished, as not all parts of the usage story are set in stone, yet.
- Adjusts CONTRIBUTING to our switch to Phabricator.
- Rewrites the LICENSE file to match our license, with GPL for the compiler and BSD for the rest. It now also contains a copy of the GPL and improves some phrasings.