Remove the `cross-domain` phrase from `README.md`

The part about `Cross-domain Ajax and Flash` from the `README.md` file
isn't accurate, as by default:

 * the `crossdomain.xml` file doesn't grant a web client — such as Adobe
   Flash Player, Adobe Reader, etc. — permission to handle data across
   multiple domains

 * the Apache server configs, do not allow cross-origin access to all
   resources, unless the user enables that behavior