Add merge_request_event to CI_PIPELINE_SOURCE list of possible values.

doc/ci/variables/predefined_variables.md

@@ -84,7 +84,7 @@ future GitLab releases.**
 | `CI_PAGES_URL`                                | 11.8   | all    | URL to GitLab Pages-built pages. Always belongs to a subdomain of `CI_PAGES_DOMAIN`.                                                                                                                                                                                                                                                                       |
 | `CI_PIPELINE_ID`                              | 8.10   | all    | The unique id of the current pipeline that GitLab CI uses internally                                                                                                                                                                                                                                                                                       |
 | `CI_PIPELINE_IID`                             | 11.0   | all    | The unique id of the current pipeline scoped to project                                                                                                                                                                                                                                                                                                    |
-| `CI_PIPELINE_SOURCE`                          | 10.0   | all    | Indicates how the pipeline was triggered. Possible options are: `push`, `web`, `trigger`, `schedule`, `api`, and `pipeline`. For pipelines created before GitLab 9.5, this will show as `unknown`                                                                                                                                                          |
+| `CI_PIPELINE_SOURCE`                          | 10.0   | all    | Indicates how the pipeline was triggered. Possible options are: `push`, `web`, `trigger`, `schedule`, `api`, `pipeline` and `merge_request_event`. For pipelines created before GitLab 9.5, this will show as `unknown`                                                                                                                                                          |
 | `CI_PIPELINE_TRIGGERED`                       | all    | all    | The flag to indicate that job was [triggered](../triggers/README.md)                                                                                                                                                                                                                                                                                       |
 | `CI_PIPELINE_URL`                             | 11.1   | 0.5    | Pipeline details URL                                                                                                                                                                                                                                                                                                                                       |
 | `CI_PROJECT_DIR`                              | all    | all    | The full path where the repository is cloned and where the job is run. If the GitLab Runner `builds_dir` parameter is set, this variable is set relative to the value of `builds_dir`. For more information, see [Advanced configuration](https://docs.gitlab.com/runner/configuration/advanced-configuration.html#the-runners-section) for GitLab Runner. |