Merge branch 'object-state-models-docs' into 'master'

Object state models docs

Object state models docs for issues and merge requests.

This is to start documenting object models, focused more on users and how they experience the product.

See merge request !7544

doc/development/README.md

@@ -38,6 +38,7 @@
 - [Rake tasks](rake_tasks.md) for development
 - [Shell commands](shell_commands.md) in the GitLab codebase
 - [Sidekiq debugging](sidekiq_debugging.md)
+- [Object state models](object_state_models.md)
 ## Databases

doc/development/object_state_models.md

+# Object state models
+
+## 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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