DOCSP-31115: Demonstrate including the Composer autoloader (#890)

This is analogous to reminding users to load the extension in php.ini. Both are also covered in the linked PHPLIB docs, but it makes sense to duplicate the information here for users that don't click through.

Author: jmikola

source/php.txt

@@ -97,6 +97,14 @@ The preferred method of installing the PHP library is with
 
    $ composer require mongodb/mongodb
 
+Once you have installed the library, ensure that your application includes
+Composer's autoloader as in the following example:
+
+.. code-block:: php
+
+   <?php
+
+   require_once __DIR__ . '/vendor/autoload.php';
 
 Additional installation instructions may be found in the
 `library documentation <https://www.mongodb.com/docs/php-library/current/tutorial/install-php-library/>`_.