Include documentation in the application

doc/transitions.markdown

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+Task transitions
+================
+
+Transitions record each movement of the tasks between columns.
+
+![Transitions](http://kanboard.net/screenshots/documentation/transitions.png)
+
+Available from the task view, you can see those information:
+
+- Date of the action
+- Source column
+- Destination column
+- Executer (user that move the task)
+- Time spent in the origin column
+
+Task transition data can also be exported from the project settings page.
+
+![Transitions Export](http://kanboard.net/screenshots/documentation/transitions-export.png)
+
+For the specified time range you will generate a CSV file that you can use with any spreadsheet software.