[documentation] Following installation steps with Symfony Flex

[o-alquimista]

For projects using Symfony Flex, only Step 1 is needed. This information is usually given in the beginning of any instruction page in the Symfony docs, but is missing for this bundle. Users who are not yet familiar with how Flex works might attempt to follow the instructions needlessly.

• I just checked debug:config and the bundle is already registered. Step 2 is done.
• The file config/routes/fos_js_routing.yaml was already placed there by the recipe. Step 3 is done.
• Step 4? Perhaps this line during installation suggests it is done too: Executing script assets:install public [OK].