Update README to signify where to add custom scripts

[ghost]

The README currently suggests adding the following "commands" to the package manifest, but it doesn't indicate where they should land:

"post-install-cmd": [
	"wp-enforcer"
],
"post-update-cmd": [
	"wp-enforcer"
]

As a total n00b using composer I thought these were some kind of built-in. It wasn't until later I realized I should've added them to the scripts section and not in the root of the manifest. I imagine a quick GH search for apps using this package will turn up the same.