Mention GE authentication with GE injection

Author: bigdaz

README.md

@@ -777,3 +777,12 @@ The `init-script` supports a number of additional configuration parameters that
 | GRADLE_ENTERPRISE_INJECTION_GE_PLUGIN_VERSION | :white_check_mark: | the version of the [Gradle Enterprise Gradle plugin](https://docs.gradle.com/enterprise/gradle-plugin/) to apply |
 | GRADLE_ENTERPRISE_INJECTION_CCUD_PLUGIN_VERSION |  | the version of the [Common Custom User Data Gradle plugin](https://github.com/gradle/common-custom-user-data-gradle-plugin) to apply, if any |
 | GRADLE_ENTERPRISE_INJECTION_PLUGIN_REPOSITORY_URL |  | the URL of the repository to use when resolving the GE and CCUD plugins; the Gradle Plugin Portal is used by default |
+
+## Authentication
+To authenticate against the Gradle Enterprise server, you will need to define an environment variable named `GRADLE_ENTERPRISE_ACCESS_KEY`.
+This is not required directly by the Gradle Enterprise injection, but will be required by the Gradle Enterprise Gradle Plugin when connecting to the server.
+
+To do this, define a [GitHub Actions secret](https://docs.github.com/en/actions/security-guides/encrypted-secrets) at the repository or organization level, and reference this as an environment variable in your workflow file.
+See the [minimal example above](#enabling-gradle-enterprise-injection) for the syntax.
+
+To generate a Gradle Enterprise Access Key, refer to the [docs for the Gradle Enterprise Gradle plugin](https://docs.gradle.com/enterprise/gradle-plugin/#manual_access_key_configuration).