add documentation for "dj mixer mode"

[wolfbiter]

as discussed in #94

• add preset 9 to the presets ppl must update (both in the README and in the QuNeo editor folder. remember this for #98)
• make a new diagram for preset 9
• add explanations for above diagram
• rename all references to "alternative dj mode" or "dj 2 mode" to "dj mixer mode"

[wolfbiter]

i think i'll make a post in the forums and up the preset to v1.2.x after adding dj mixer mode and finishing #98