Extension installation: Reference the composer config in config.yml for advanced devs

When attempting to develop locally, it is impossible to load a `dev` extension into Bolt without either setting it to a `stable` release (which is obviously not good for a new extension) or editing the composer settings in `config.yml`. Myself and [others](https://github.com/bolt/bolt/issues/6955#issuecomment-325801116) have hit a wall attempting to manually set `minimum-stability:true` in `extensions/composer.json`, only to have it constantly revert back to `true`.

I understand that the composer config vars in `config.yml` are not for everyone, and ideally should not be utilized most of the time. But they are there for a reason, and I think it's worth at least a mention in the [installing new extensions](https://docs.bolt.cm/3.5/extensions/introduction#installing-new-extensions) section.