add spatie/menu to navigation section (#1019)

README.md

 *Tools for building navigation structures.*
 
 * [KnpMenu](https://github.com/KnpLabs/KnpMenu) - A menu library.
+* [Menu](https://github.com/spatie/menu) - Very flexible menu library with fluent interface.
 
 ### Asset Management
 *Tools for managing, compressing and minifying website assets.*