Prepare for v5.18 release

Author: codezninja

CHANGELOG.md

@@ -1,5 +1,7 @@
 # Unreleased
 
+# v5.18
+
 - [Issue #214](https://github.com/manheim/terraform-pipeline/issues/214) CredentialsPlugin: support more than username/password (Deprecate `withBuildCredentials`)
 - [Issue #399](https://github.com/manheim/terraform-pipeline/issues/399) Add pull request templates, with checkboxes for necessary tasks
 - [Issue #309](https://github.com/manheim/terraform-pipeline/issues/309) Add the ability to specify terraform-pipeline starting execution workspace directory