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.
well, the best practice in the doc was written at a time where symfony.com was expected to have a repository of bundles with their doc being rendered (KnpBundles appeared as a third-party solution to this problem because the official tool announced by Fabien was not yet there, and never became a reality given we then choose composer meaning that KnpBundles and Packagist can probably be enough and people can use libraries which are not bundles). The de-facto standard then became Markdown because everyone is using Github (well, mostly at least) and it used to suck at rendering rST (3 years ago when building the Symfony2 ecosystem, the ``code-block`` directive was not supported by their rST renderer for instance) while Markdown was well rendered