added images for merge request instruction

doc/source/vcs.rst

@@ -214,8 +214,15 @@ Workflow with ``dev`` branch
     To https://csst-tb.bao.ac.cn/code/csst-l1/csst_proto.git
        d9f64d3..bbed3fa  main -> main
 
-.. note::
-    As a developer, you can also create a merge requrest to ask code reviewers to merge your changes on GitLab.
-    Click ``Merge requests`` tag, click ``
-
-.. image:: vcs/merge_request.png
+As a developer, you can also create a merge requrest to ask code reviewers to merge your changes on GitLab.
+
+First, click ``Merge requests`` tag, click ``
+    .. image:: vcs/merge_request.png
+Second, select source branch and target branch, create a new merge request.
+    .. image:: vcs/create_merge_request.png
+The code reviewers should then view the chagnes, and decide whether the changes should be approved.
+    .. image:: vcs/create_merge_request.png
+If the changes are accepted, approve and merge the changes into `main` branch.
+    .. image:: vcs/merge_approve.png
+After the changes have been merged, code reviewers should leave comment on the changes.
+    .. image:: vcs/merged.png
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