Document IS_GITLAB_EE environment variable

doc/development/ee_features.md

@@ -19,6 +19,11 @@ CE specs should remain untouched as much as possible and extra specs
 should be added for EE. Licensed features can be stubbed using the
 spec helper `stub_licensed_features` in `EE::LicenseHelpers`.
 
+You can force Webpack to act as CE by either deleting the `ee/` directory or by
+setting the [`IS_GITLAB_EE` environment variable](https://gitlab.com/gitlab-org/gitlab-ee/blob/master/config/helpers/is_ee_env.js)
+to something that evaluates as `false`. The same works for running tests
+(for example `IS_GITLAB_EE=0 yarn jest`).
+
 [ee-as-ce]: https://gitlab.com/gitlab-org/gitlab-ee/issues/2500
 
 ## Separation of EE code