Add a readme with documentation on the scripts in the bin folder

[andreicuceu]

As a lot of the work in picca gets done by scripts in the bin folder, it would be nice to have a readme explaining what each of them does, and maybe guiding people on what is best practice for certain use cases. Also some description of the input parameters (I know there is --help, but a centralized documentation is definitely better) and examples would probably be good.

[Waelthus]

a central doc is envisioned for after Lya-1pt