Pickling

[lilyminium]

Add a pickling section.

Universe, Readers can get pickled.

[yuxuanzhuang]

Also related to #150.
I wrote one a while back. Let me know if you think there's anything else that needs to be added to this section and I would love to help with it:)
https://userguide.mdanalysis.org/2.0.0-dev0/reading_and_writing.html?highlight=serialization#pickling