Start of Popper primer.

[rpgoldman]

Started to write some notes, which might be of general usefulness, based on the questions I posted in the issues, and the answers I received.

Not ready for prime time, but would welcome comments on the contents and feedback about whether this would be helpful.

[andrewcropper]

Hi, it might be best to write such a primer in the wiki section, which I just created here: https://github.com/logic-and-learning-lab/Popper/wiki

Doing so should reduces the number of pull requests and, I think, make it easier for other people to edit.