Improve illustration/example of how to share code segment context

[bielsnohr]

I think this could be tightened up a bit and made more concrete. At the moment it is a touch on the vague side. I am thinking of something like the specific examples we were writing a while ago with the individual step recipes. Rough first pass:

Lisa is writing a library for molecular interactions and has a class in her code to represent a molecule. She has recently added a few methods like get_position(), get_velocity(), find_nearest_neighbours() and she wants to make sure the code is readable and conforms to Python standards. When she shares the class and methods with her reviewer, this is the context she provides: ... (fill in)

Originally posted by @bielsnohr in #50 (comment)