Update README.md

For using the flags inline with text add the classes `.flag-icon` and `.flag-icon-xx`

(where `xx` is the

[ISO 3166-1-alpha-2 code](http://www.iso.org/iso/country_names_and_code_elements) 

of a country) to an empty `<span>`. If you want the flag to have the squared version 

then also add the class `flag-icon-squared`. Example:

of a country) to an empty `<span>`. If you want to have a squared version flag

then add the class `flag-icon-squared` as well. Example:

    <span class="flag-icon flag-icon-gr"></span>

    <span class="flag-icon flag-icon-gr flag-icon-squared"></span>