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.
Re: Screen shoots.
Please, do not add your screen-shoot images (*.png) yet. Just add the ``.. figure::`` code but leave out the actual image. This will throws an non-critical error message when compiling, but at the same time saves you from adding an extra TODO item. 

This is not only with this PR. To avoid duplication of work, i really like to add the screen-shoots at the end of the release process, when we have the final form on the images (#8). Also screen-shoots should be done consistent on **one** OS, not mixed up. I`ll volunteer to do this with Mac OSX, just like with the previous release.