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.
@WouterJ it's true that this started as GitHub-specific but it's generally accepted now. In addition, 99.99% of the bundles are published on GitHub. And another compelling reason: if you are able to develop a Symfony bundle, you definitely know how to manage all these Markdown formatting issues.