added section matlab gui

* added section about matlab gui elements
* added paragraph about matlab documentary
I think information on *"where MATLAB should find my functions and scripts?"* is currently irrelevant to learners. They've just opened up MATLAB for the first time. What *is* relevant is *"what directory is MATLAB currently looking at?"*. 

I think telling learners about setting and changing the *current directory* is sufficient at this time, and a note on adding other directories to the search path is appropriate at a later stage.