Add 'About' or 'What is WebID' link to signup screen

[dmitrizagidulin]

Here's the current UI that a user encounters when loading the Signup app (taken from http://databox.me):

- Add explanatory wording, to the effect of "This will set you up with a WebID identity and storage account." - Link the WebID to an explanation - Add an `About` link at the bottom, to a page explaining why one should sign up, what WebID is, and what the user should do (like which apps to try) once they create an account. - Add a github 'source code' link to the repo