update contributing.md

CONTRIBUTING.md

@@ -41,6 +41,10 @@ Here are a few things you can do that will increase the likelihood of your pull
 - Keep your change as focused as possible. If there are multiple changes you would like to make that are not dependent upon each other, consider submitting them as separate pull requests.
 - Write a [good commit message](http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html).
 
+## Licensed
+
+This repository uses a tool called [Licensed](https://github.com/github/licensed) to verify third party dependencies. You may need to locally install licensed and run `licensed cache` to update the dependency cache if you install or update a production dependency. If licensed cache is unable to determine the dependency, you may need to modify the cache file yourself to put the correct license. You should still verify the dependency, licensed is a tool to help, but is not a substitute for human review of dependencies.
+
 ## Resources
 
 - [How to Contribute to Open Source](https://opensource.guide/how-to-contribute/)