Add README.md file

Add basic info. This file is automatically displayed on GitHub, and is
kind of de-facto standard for project documentation.

BUG=chromium:415466
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/657423002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24731 ce2b1a6d-e550-0410-aec6-3dcde31c8c00