Use smart quotes in README.md

Why?

- smart quotes are prettier 
- they also read easier
- the document was using a mix of smart- and straight-quotes anyway
- (It doesn’t matter in this document, but…) in documents with code examples, smart quotes in comments and body text keep text editors from confusing them with quotes in code
- smart quotes add polish (and credibility)—important for “call to action” documents like this ☺