Add advanced Gitlab CI example (lint all shell scripts in git project)

GitLab-CI.md

@@ -7,3 +7,16 @@ test:
     script: 
     - shellcheck testScript.sh
 ```
+
+Here is a CI job that will lint all shell scripts in a git repository:
+
+```yaml
+test:
+    image: koalaman/shellcheck-alpine:latest
+    stage: test
+    before_script:
+    - apk update
+    - apk add git
+    script:
+    - git ls-files --exclude='*.sh' --ignored | xargs shellcheck
+```