Add coach and working-alone notes

[ialja]

There will probably be two ways for using this tutorial:

• one at a workshop with a coach and
• the other at home (or wherever), doing it alone.

We still need to provide better support for both modes. For the coach mode, short hints for the coach about basic concepts that should be explained at certain points (Rails Girls style) and for the working-alone mode, "More info" boxes with simple explanations and links to websites, where you can find out more if you want to understand how something works.