Generated README mention all packageManagers instead of the one selected in the CLI prompt

[mrleblanc101]

Shouldn't the generated README only mention the selected packageManager when using the CLI ?

[OrbisK]

To achieve this, we need to make slight modifications to the template's README.md file. However, templates are also used for other things, such as StackBlitz Starter.

I think we could add some comments to README.md, such as:

<-- pm_install -->
```bash
  <install commands>
```

This allows us to identify (regex) the code block and replace it without breaking it.

cc @danielroe wdyt?