move contents of readme into library docs

BUG=
R=ajohnsen@google.com

Review URL: https://codereview.chromium.org//113733002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@31104 260f80e4-7a28-3924-810f-c04153c831b5
2 files changed