[docs] Add notice of project closure to README
[docs] Add notice of project closure to README

Make it clear that the project is not being developed anymore with a notice at the beginning of the README file.
crosswalk-project/crosswalk,crosswalk-project/crosswalk,crosswalk-project/crosswalk,crosswalk-project/crosswalk,crosswalk-project/crosswalk,crosswalk-project/crosswalk,crosswalk-project/crosswalk,crosswalk-project/crosswalk