Add documentation about Kanban

docs/what-is-kanban.markdown

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+What is Kanban?
+===============
+
+Kanban is a methodology originally developed by Toyota to be more efficient.
+
+There is only two constraints imposed by Kanban:
+
+- Visualize your workflow
+- Limit your work in progress
+
+Visualize your workflow
+-----------------------
+
+- Your work is visualized on a board, you have a clear overview of your project
+- Each column represent a step in your workflow
+
+Bring focus and avoid multitasking
+----------------------------------
+
+- Each phase can have a work in progress limit
+- Limits are great to identify bottlenecks
+- Limits avoid working on too many tasks in the same time
+
+Measure performance and improvement
+-----------------------------------
+
+Kanban uses lead and cycle times to measure performance:
+
+- **Lead time**: Time between the task is created and completed
+- **Cycle time**: Time between the task is started and completed
+
+By example, you may have a lead time of 100 days and only have to work 1 hour to complete the task.