Add README file to the mime package

Also update CHANGELOG after last change.

R=kustermann@google.com, wibling@google.com
BUG=http://dartbug.com/22442

Review URL: https://codereview.chromium.org//936733004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/mime@43841 260f80e4-7a28-3924-810f-c04153c831b5