add advisory note to changelog

CHANGELOG.md

@@ -1,7 +1,9 @@
 # 0.13.3 (Unreleased)
 
 SECURITY:
- - `tfinstall`: The HashiCorp PGP signing key has been rotated (HCSEC-2021-12). This key is used to verify downloaded versions of Terraform. We recommend all users of terraform-exec upgrade to v0.13.3 for this security fix. [GH-166]
+ - `tfinstall`: The HashiCorp PGP signing key has been rotated ([HCSEC-2021-12](https://discuss.hashicorp.com/t/hcsec-2021-12-codecov-security-event-and-hashicorp-gpg-key-exposure/23512)). This key is used to verify downloaded versions of Terraform. We recommend all users of terraform-exec upgrade to v0.13.3 for this security fix. [GH-166]
+
+N.B. Versions of terraform-exec prior to v0.13.3 will continue to verify older versions of Terraform (up to and including v0.15.0) for a limited period. **Installation of Terraform using older versions of terraform-exec will stop working soon, and we recommend upgrading as soon as possible to avoid any interruption.**
 
 # 0.13.2 (April 06, 2021)