Document how to update repository (pull new changes)

[krassowski]

It may not be obvious that a feature to update the repository (pull new changes) exists and how it works because this button only shows up after initial cloning and is only active after new changes are made to the remote repository.

Fuhrer, it is not clear how it works - a link to nbgitpuller docs might be enough here.

[krassowski]

This should probably be done after revamp of the UX/design of tiles is completed to make sure the docs reflect the final state #22