Add good tutorial for phase retrieval

[david-hoffman]

To show why this note exists and what users can expect.

NOTE: If all that is needed is phase, e.g. for adaptive optical correction, then most normal ways of estimating the background should be sufficient and you can use the phase_only keyword. However, if you want to properly model your PSF for something like deconvolution then you should be aware that the magnitude estimate is incredibly sensitive to the background correction applied to the data prior to running the algorithm, and multiple background methods/parameters should be tried.