Adds docs on tooltips zenorocha#423

Author: zenorocha

readme.md

@@ -119,7 +119,13 @@ clipboard.on('error', function(e) {
 });
 ```
 
-For a live demonstration, open this [site](https://clipboardjs.com/) and just your console :)
+For a live demonstration, go to this [site](https://clipboardjs.com/) and open your console.
+
+## Tooltips
+
+Each application has different design needs, that's why clipboard.js does not include any CSS or built-in tooltip solution.
+
+The tooltips you see on the [demo site](https://clipboardjs.com/) were built using [GitHub's Primer](http://primercss.io/tooltips/). You may want to check that out if you're looking for a similar look and feel.
 
 ## Advanced Options