Created Docking (markdown)

Docking.md

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+## About Docking
+
+#### Where to find it?
+
+Docking is currently available in the [docking](https://github.com/ocornut/imgui/tree/docking) branch. This is a well maintained branch and most large teams have been using this branch for a while. It is safe and recommended to use that branch.
+
+#### How can I enable docking?
+
+`ImGui::GetIO().ConfigFlags |= ImGuiConfigFlags_DockingEnable;`
+
+That's pretty much it.
+There are 4 additional docked related configuration flags in the io structure which you may toy with.
+
+#### Why is not merged to master?
+
+(Please note that the following paragraphs have been written by a overly cautious person)
+
+Several advanced features, such as tight interaction with native multi-viewports, or the DockBuilder are still expecting api changes. I am also not particularly happy with how some aspects of the code are current implemented, and considering to maybe rewrite all of docking from scratch a third time! 
+
+However, you can benefit from a lot of docking features without being impacted by any of this. **The largest amount of interactions you'll have with the docking system are a user level and not at API level**. By just enabling the config flag above and calling 1-2 functions you can benefit from 90% of Docking features. Even the hypothetical full rewrite of Docking system is not expected to impact most users. API that are most at risk of changing are hidden in `imgui_internal.h` for this reason, and even so, if they do change, we'll make reasonable effort to document the reasoning the update path. 
+
+In addition, Docking will only move forward with feedback from users, so the more people using it, the closer we are to a reach mergeable version. TL;DR; is totally fine to use for most users.
+
+## Usage Guide