Tidy up the README

After getting dartlang.org up and running locally, after using the helpful steps in the README, I thought I'd update the steps. This PR:

* changes most URLs to be links (saving horizontal space)
* nests lists less
* adds links for Windows installers
* Fixes the codeblocks inside lists (8-column rule)
* `Monospaces` directory names in text
* Uses a 100-column limit for text, but this is just my personal preference