Update object_state_models.md [ci skip]

doc/development/object_state_models.md

+# Object state models
+
+## Contents
+* [Diagrams](#diagrams)
+* [Legend](#legend)
+* [Issue](#issue)
+* [Merge request](#merge-request)
+
+## Diagrams
+[GitLab object state models](https://drive.google.com/drive/u/3/folders/0B5tDlHAM4iZINmpvYlJXcDVqMGc)
+
+---
+
+## Legend
+![legend](img/state-model-legend.png)
+
+--- 
+
+## Issue
+[`app/models/issue.rb`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/models/issue.rb)
+![issue](img/state-model-issue.png)
+
+--- 
+
+## Merge request
+[`app/models/merge_request.rb`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/models/merge_request.rb)
+![merge request](img/state-model-merge-request.png)
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