Merge pull request #20 from claudioc/patch-1

Replace w3schools with a better reference

README.md

@@ -160,8 +160,7 @@ $ pup < robots.html a slice{:3}
 
 ## Implemented Selectors
 
-For further examples of these selectors head over to [w3schools](
-http://www.w3schools.com/cssref/css_selectors.asp).
+For further examples of these selectors head over to [MDN](https://developer.mozilla.org/en-US/docs/Web/CSS/Reference).
 
 ```bash
 cat index.html | pup .class