Update docs and changelog

Author: duckpuppy

CHANGELOG.md

@@ -1,6 +1,8 @@
 # Unreleased
 
 * [Issue #332](https://github.com/manheim/terraform-pipeline/issues/332) Upgrade from Junit 4 to 5.7, upgrade hamcrest from 1.3 to 2.2, remove junit-hierarchicalcontextrunner dependency
+* [Issue #344](https://github.com/manheim/terraform-pipeline/issues/344) Add PLAN_ONLY parameter to PlanOnlyPlugin
+  * **BREAKING CHANGE** This change is a breaking change. Prior to this update, applying the PlanOnlyPlugin would restrict the pipeline to only running `terraform plan`. This update changes behavior to simply providing a `PLAN_ONLY` boolean parameter that can be set to restrict the build behavior. It defaults to `false`.
 
 # v5.14
 

docs/PlanOnlyPlugin.md

@@ -1,6 +1,6 @@
 ## [PlanOnlyPlugin](../src/PlanOnlyPlugin.groovy)
 
-Enable this plugin to change pipeline functionality to `terraform plan`.
+Enable this plugin to add a parameter to the build which will restrict pipeline functionality to `terraform plan` only.
 
 ```
 // Jenkinsfile