Effects

See Issue #11 

Introduce the effects in the UI section. The text is currently very short.

I have also added a section for the various effects in the `djing-with-mixxx` chapter. Each effect can be explained in there. Including the controls.

Is anything missing to explain the Effects framework?
Hm I don't know about "Fade a song out".  I know DJs who use a high-pass as a very precise bass kill knob in a full mix

"The filter changes the tone of the music by allowing only high or low frequencies to pass through."