Added documentation around ProfilingPanel and problems with the middlwares

Closes #497
This isn't very clear because instrumenting can be read as a synonym for profiling.

I would suggest something along these lines: "if you run into this issues, then you should either disable the ProfilingPanel or move DebugToolbarMiddleware to the end of MIDDLEWARE_CLASSES. If you do the latter, then the debug toolbar won't track the execution of other middleware."