Add documentation about the license encryption keys

doc/development/licensing.md

@@ -48,6 +48,12 @@ For all of the above, please include `--why "Reason"` and `--who "My Name"` so t
 
 More detailed information on how the gem and its commands work is available in the [License Finder README](https://github.com/pivotal/LicenseFinder).
 
+## Encryption keys
+
+If your license was created in your local development or staging environment for Customers Portal or License App, an environment variable called `GITLAB_LICENSE_MODE` with the value `test` needs to be set to use the correct decryption key.
+
+Those projects are set to use a test license encryption key by default.
+
 ## Additional information
 
 Please see the [Open Source](https://about.gitlab.com/handbook/engineering/open-source/#using-open-source-libraries) page for more information on licensing.