Add Key-notation preferences

Addresses #2 

This adds a short paragraph about the key detection settings. The search operators are already included. I was wondering if we should add a chapter that explains why keys are useful and how to use them during a gig.
> Should I copy the image from our main repository and show that inline?

Link to the user interface section instead, e.g. 
```rest
To enable :term:`Keylock`, click the :guilabel:`Keylock` button in the :ref:`interface-button-grid`.
```

Note:
If you want cross-references to terms in the glossary using the ``:term:`` role, add them to ``glossary.rst``, see http://sphinx-doc.org/markup/inline.html#cross-referencing-other-items-of-interest