Add documentation for GITLAB_FEATURES env var for CI/CD

see https://gitlab.com/gitlab-org/gitlab-ce/issues/40994

doc/ci/variables/README.md

@@ -84,6 +84,7 @@ future GitLab releases.**
 | **GITLAB_USER_EMAIL**           | 8.12   | all    | The email of the user who started the job |
 | **GITLAB_USER_LOGIN**           | 10.0   | all    | The login username of the user who started the job |
 | **GITLAB_USER_NAME**            | 10.0   | all    | The real name of the user who started the job |
+| **GITLAB_FEATURES**             | 10.6   | all    | The comma separated list of licensed features available for your instance and plan |
 | **RESTORE_CACHE_ATTEMPTS**      | 8.15   | 1.9    | Number of attempts to restore the cache running a job |
 
 ## 9.0 Renaming