Add info about external dependencies
Add info about external dependencies

Thanks to #simonmorley

Issue:
https://github.com/Houdini/two_factor_authentication/issues/5
dahal/two_factor_authentication,monfresh/two_factor_authentication,JamesHarker/two_factor_authentication,dahal/two_factor_authentication,fonji/two_factor_authentication_root,fonji/two_factor_authentication_root,JamesHarker/two_factor_authentication,dahal/two_factor_authentication,Houdini/two_factor_authentication,fonji/two_factor_authentication_root,benjaminwols/two_factor_authentication,Houdini/two_factor_authentication,alphagov/two_factor_authentication,benjaminwols/two_factor_authentication,monfresh/two_factor_authentication,alphagov/two_factor_authentication,benjaminwols/two_factor_authentication,alphagov/two_factor_authentication,Houdini/two_factor_authentication,monfresh/two_factor_authentication