Comment fix for submitted patch: https://codereview.chromium.org/267423005
This patch renames README file to README.chromium and also updated
command to generate subset font

BUG=372502

Review URL: https://codereview.chromium.org/289283010

git-svn-id: svn://svn.chromium.org/blink/trunk@174790 bbb929c8-8fbe-4397-9dbb-9b2b20218538