Adds trigger to Trigger tutorial

Closes #1328.
 * Adds a trigger for no_mushrooms
 * Updated the text to explain it to the tutee
 * Updated text to make the example clearer about directories
Please run:
```
tidy -i -c -utf8 -wrap 80 -m  doc/rose-rug-advanced-tutorials-trigger.html
```
to wrap the text to 80 characters. This will also add some annoying changes
at the top of the file which should be undone.