Added standard bundle installation instructions

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | -
| Fixed tickets | #4157

Note: if you think that this draft is awful, please say it so and we'll rewrite it from scratch.
... bundle by adding the following line in the `app/AppKernel.php` file of your project: