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.
You are right. RST is the official format for the bundle documentation, but 99% of bundles hosted on GitHub use `README.md` instead of `README.rst`. I've switched the template to Markdown.