removed installation instructions as submodule

[cheolhom]

not sure why it was removed, but now it just says
either use git clone git://github.com/technoweenie/restful-authentication.git restful_authentication
or rename the plugin’s directory to be restful_authentication after fetching it.

previously had instructions to add as submodule if using git.
git submodule add git://github.com/technoweenie/restful-authentication.git vendor/plugins/restful_authentication