add help page

This is a draft of a new help page:
![tutorials](https://f.cloud.github.com/assets/3798614/968341/935f414e-059d-11e3-9ecf-e3757d3d2cb2.png)
![tutorials_signing_up](https://f.cloud.github.com/assets/3798614/968342/964785b0-059d-11e3-8071-4ae626cd0276.png)


[loomio discussion](https://www.loomio.org/discussions/5347)