Document creating social directory

[the-overengineer]

Document creating a social directory via Client.createDirectory, using Twitter as an example.
A real-world example, tested manually, verified via dev console (api.stormpath).

Fixes #577

[the-overengineer]

Makes mention of #578. Not sure if sufficient.

[coveralls]

Coverage decreased (-0.08%) to 89.414% when pulling 4ac007a on documentation/twitter-directory into bcc4597 on master.

[coveralls]

Coverage decreased (-0.08%) to 89.414% when pulling 4ac007a on documentation/twitter-directory into bcc4597 on master.

[coveralls]

Coverage decreased (-0.08%) to 89.414% when pulling 4ac007a on documentation/twitter-directory into bcc4597 on master.

[coveralls]

Coverage decreased (-0.08%) to 89.414% when pulling 4ac007a on documentation/twitter-directory into bcc4597 on master.

[coveralls]

Coverage remained the same at 89.492% when pulling 4ac007a on documentation/twitter-directory into bcc4597 on master.

[robertjd]

+1, thanks!