Add Doctrine ORM installation instructions.

This best practices page assumes Doctrine is available and, since it is not, the reader is given a little extra information and a link to know how to make it available and be able to continue through the rest of the examples. This makes it consistent with the Data Fixtures description later on the page.

Author: LavaSlider

best_practices/business-logic.rst

@@ -191,6 +191,15 @@ library or strategy you want for this.
 
 In practice, many Symfony applications rely on the independent
 `Doctrine project`_ to define their model using entities and repositories.
+
+Doctrine support is not enabled by default in Symfony. So to use Doctrine
+as shown in the examples below you will need to install `Doctrine ORM support`_
+by executing the following command:
+
+.. code-block:: terminal
+
+    $ composer require "symfony/orm-pack"
+
 Just like with business logic, we recommend storing Doctrine entities in the
 AppBundle.
 
@@ -357,6 +366,7 @@ Next: :doc:`/best_practices/controllers`
 
 .. _`full definition`: https://en.wikipedia.org/wiki/Business_logic
 .. _`Doctrine project`: http://www.doctrine-project.org/
+.. _`Doctrine ORM support`: https://symfony.com/doc/current/doctrine.html
 .. _`fixture class`: https://symfony.com/doc/current/bundles/DoctrineFixturesBundle/index.html#writing-simple-fixtures
 .. _`PSR-1`: https://www.php-fig.org/psr/psr-1/
 .. _`PSR-2`: https://www.php-fig.org/psr/psr-2/