minor #8924 Install annotations on the top of routing (weaverryan)

This PR was merged into the 4.0 branch.

Discussion
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Install annotations on the top of routing

Reported by a user somewhere on an old recipe PR :). We didn't show it here originally, because if you go through the "Getting Started" in order, you will install it in "Page creation". But, each chapter needs to also stand on its own.

Commits
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43428c4 adding install details about annotations in the routing chapter

Author: weaverryan

routing.rst

@@ -20,6 +20,12 @@ the change is simple.
 Creating Routes
 ---------------
 
+First, install the annotations package:
+
+.. code-block:: terminal
+
+    $ composer require annotations
+
 A *route* is a map from a URL path to a controller. Suppose you want one route that
 matches ``/blog`` exactly and another more dynamic route that can match *any* URL
 like ``/blog/my-post`` or ``/blog/all-about-symfony``: