README.md: reorganize the git section

1) group entries in two sets: github and non-github

2) for each suggested reading provide a brief explanation
README.md: correct nit
README.md: fix another nit
README.md: move REST & CRUD to Angular.js section

Antonio asked me to anticipate REST & CRUD (and I
did that on Monday); for correctness, move these
two concepts on the Angular.js section.
Merge pull request #10 from bassosimone/patch-1

A bunch of related README.md changes