docs/contributing: Add Resource Policy Support section, fix tagging documentation heading level (#13322)
docs/contributing: Add Resource Policy Support section, fix tagging documentation heading level (#13322)

Reference: https://github.com/terraform-providers/terraform-provider-aws/pull/11960#pullrequestreview-409674625
terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws