Improve README - Githubissues

[ ] It starts off assuming you know what a PPL is (not even expanding the acronym) and what it's for.
[ ] It assumes you know what a CFG is and what CKY is.
[ ] Explain what a sum-product is.
[ ] Give a minimal invocation example using -z
[ ] Explain how to use the fggs library to compute a sum-product.
[ ] Explain how to define an extern distribution (e.g., for von_neumann.ppl).

diprism / perpl