Added information about spread attributes

Added some information about spread attributes in the docs

Author: Even Stensberg

lessons/05-active-links/README.md

@@ -51,6 +51,10 @@ primary navigation links need to know. It's useful to wrap those so you
 don't have to remember what your `activeClassName` or `activeStyle` is
 everywhere.
 
+We will use an spread attribute here, the three dots. It clones our props 
+and in this use case it clones `activeClassName` to our desired component for
+us to benefit from.
+
 Create a new file at `modules/NavLink.js` that looks like this:
 
 ```js