Document merge request child pipelines

doc/ci/parent_child_pipelines.md

@@ -80,6 +80,52 @@ microservice_a:
     strategy: depend
 ```
 
+## Merge Request child pipelines
+
+To trigger a child pipeline as a [Merge Request Pipeline](merge_request_pipelines/index.md) we need to:
+
+- Set the trigger job to run on merge requests:
+
+```yaml
+# parent .gitlab-ci.yml
+microservice_a:
+  trigger:
+    include: path/to/microservice_a.yml
+  rules:
+    - if: $CI_MERGE_REQUEST_ID
+```
+
+- Configure the child pipeline by either:
+
+  - Setting all jobs in the child pipeline to evaluate in the context of a merge request:
+
+    ```yaml
+    # child path/to/microservice_a.yml
+    workflow:
+      rules:
+        - if: $CI_MERGE_REQUEST_ID
+
+    job1:
+      script: ...
+
+    job2:
+      script: ...
+    ```
+
+  - Alternatively, setting the rule per job. For example, to create only `job1` in
+    the context of merge request pipelines:
+
+    ```yaml
+    # child path/to/microservice_a.yml
+    job1:
+      script: ...
+      rules:
+        - if: $CI_MERGE_REQUEST_ID
+
+    job2:
+      script: ...
+    ```
+
 ## Limitations
 
 A parent pipeline can trigger many child pipelines, but a child pipeline cannot trigger