Update `enforcement` documentation

[stevenyslee]

TL;DR

The README documentation should replace instances of enforce with enforcement and clarify that if an allow or deny list is included that the value will be ignored. The Inputs table should also be updated to say that the enforcement field is required

Expected behavior

No response

Observed behavior

No response

Terraform Configuration

rules = [
    {
      enforcement = null // required
      allow = [
        "@someTestOrg.net",
      ]
      deny = []
      conditions = [
        {
          description = "A condition"
          expression  = "resource.matchTag('123456789/tag-key', 'val1')"
          title       = "Allow resource"
      }, ]
    }]

Terraform Version

1.2.3

Additional information

No response