Modify README.md and add the LICENSE file.

README.md needs to be refined with below changes:
1. Add the introduction part.
2. Add the HOWTO part to help users how to build sample web apps into
Android APKs. Currently Tizen is a missing part.
3. Add the license part.

Besides, the LICENSE file is the same as crosswalk.
Merge pull request #20 from yongsheng/master

Modify README.md and add the LICENSE file.