minor #10559 Update best_practices.rst (ThomasLandauer)

This PR was submitted for the 4.1 branch but it was merged into the 4.2 branch instead (closes #10559).

Discussion
----------

Update best_practices.rst

Example of `autoload` in `composer.json`

Commits
-------

970c26d Update best_practices.rst

Author: javiereguiluz

bundles/best_practices.rst

@@ -517,7 +517,9 @@ The ``composer.json`` file should include at least the following metadata:
 
 ``autoload``
     This information is used by Symfony to load the classes of the bundle. It's
-    recommended to use the `PSR-4`_ autoload standard.
+    recommended to use the `PSR-4`_ autoload standard. Use the namespace as key,
+    and the location of the bundle's main class (relative to ``composer.json``)
+    as value, example: ``"autoload": { "psr-4": { "JohnSmith\\BlogBundle\\": "" } }``
 
 In order to make it easier for developers to find your bundle, register it on
 `Packagist`_, the official repository for Composer packages.