README should explain order of precedence for custom paths

[joachim-n]

If you want to set a custom path for a single package whose type as a whole also has an installer path, it's essential for it to come first in the list.

In other words, the first possible path that matches for a package is the one that is considered. This should be mentioned in the README.

For example, this doesn't work:

        "installer-paths": {
            "web/modules/contrib/{$name}": ["type:drupal-module"],
            "web/special/{$name}": [
                "drupal/honeypot"
            ]
        },

but this does:

        "installer-paths": {
            "web/special/{$name}": [
                "drupal/honeypot"
            ],
            "web/modules/contrib/{$name}": ["type:drupal-module"]
        },