Add information about global route patterns

There's this really nice feature that lets you set global regular expressions for all your routes:

<!-- language: lang-php -->

    Route::pattern('id', '\d+');
    Route::pattern('slug', '[\da-zA-Z-]+');
    // etc.

I learnt about them from [Aaron Kuzemchak's post](http://kuzemchak.net/blog/entry/laravel-quick-tip-global-route-constraints) last week, and figured it's a pity that it's not in the docs...