Added help page to app 

This resolves [issue 680](https://github.com/hypothesis/h/issues/680)

There is a link in the username menu.
Clicking it opens a new tab. 
The tab is pointed to our app at /docs/help.
This route is recognized by our app and the document is served. 

You can test this at: helpv1.dokku.hypothes.is/docs/help