Document Behringer B-CONTROL Deejay BCD3000 controller

[Holzhaus]

Starting with Mixxx 2.3, all controller mappings are now supposed to be documented in the Manual. This is necessary so that other users know how to use this mapping.

This controller mapping is not documented properly at the moment.

More Information: https://github.com/mixxxdj/mixxx/wiki/Contributing%20mappings#documenting-the-mapping Example of a documented mapping: https://manual.mixxx.org/2.3/en/hardware/controllers/roland_dj_505.html