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

[michaelborn]

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 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 section.

[michaelborn]

Or perhaps, just "Also see 'Extensions' under 'Advanced Configuration'".

Then create a new Advanced Configuration section at https://docs.bolt.cm/3.5/configuration/advanced/extensions