Use `$routes->fallbacks()` instead of repeat code

On RouterBuilder we have a `fallbacks()` method that does exactly what we are doing on routes.php.

I think it's better to use the method because this way users known its existence and will use it instead of copy-pasting two lines of code.
Added comment about `fallbacks` method
Merge pull request #86 from elboletaire/patch-1

Use `$routes->fallbacks()` instead of repeat code