Document PYSIDE-315 implications

Document about why dynamically adding signals and slots (on signal connection for instance) will cause issues both in Qt and PySide, and might lead to hard-to-debug issues and crashes.

Main takeaways:
1) Always decorate slots with @QtCore.Slot()
2) Don't dynamically add / remove signals or slots after a connection is made to any signal or slot.
3) Do not create signals in __init__ methods, the mySignal = QtCore.Signal() definitions should be as class members of the custom class.