Added markdown link instead of using plain url in Collection readme

src/Collection/README.md

@@ -25,6 +25,6 @@ you have in your application as well.
 
 ## Documentation
 
-Please make sure you check the official documentation at http://book.cakephp.org/3.0/en/core-libraries/collections.html
+Please make sure you check the [official documentation](http://book.cakephp.org/3.0/en/core-libraries/collections.html)