docs: Update README.md

Some improvement from #123

README.md

@@ -32,10 +32,14 @@ To control module's behavior, change variables' values regarding the following:
 
 - `constraint`: set this variable with the [constraint value](https://cloud.google.com/resource-manager/docs/organization-policy/org-policy-constraints#available_constraints) in the form `constraints/{constraint identifier}`. For example, `constraints/serviceuser.services`
 - `policy_type`: Specify either `boolean` for boolean policies or `list` for list policies. (default `list`)
-- `policy_for`: set one of the following values to determine where the policy is applied:
+- Policy Root: set one of the following values to determine where the policy is applied:
   - `organization_id`
   - `project_id`
   - `folder_id`
+- `policy_for`: Specify the hierarchy level where the policy is applied. Can be **one of** the following values.
+  - `organization`
+  - `folder`
+  - `project`
 - `exclude_folders`: a list of folder IDs to be excluded from this policy. These folders must be lower in the hierarchy than the policy root.
 - `exclude_projects`: a list of project IDs to be excluded from this policy. They must be lower in the hierarchy than the policy root.
 - Boolean policies (with `policy_type: "boolean"`) can set the following variables: