Fairly extensive changes to README.md

* Re-org'd the intro to flow a bit better
* Made sure each service is linked to its homepage
* Linked a few terms to descriptive pages, just to be all helpful-like
* Standardized the formatting of the suggested services ("Pros…Cons…",
bullet points)

Really, all I'd intended to do was add links to the services but things
got a little out of hand once I started editing the file. =)