Qt5 Migration
Porting C++ Applications to Qt5 - Simple overview
Qt5 C++ API Changes - review all parts of Mixxx that use these and make the recommended changes
This is the Mixxx Wiki - 
Mixxx is a free and open-source DJ software.
Manual
Hardware Compatibility
Reporting Bugs
Getting Involved
Contribution Guidelines
Coding Guidelines
Using Git
Developer Guide
Contributing Mappings
Mixxx Controls
MIDI Scripting
Components JS