Draft: add README files to explain the new structure

src/Mod/Draft/draftguitools/README.md

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+2020 February
+
+These files define the GuiCommands, that is, actions called in a graphical
+way, either buttons, menu entries, or context commands.
+
+These tools should be split from the big `DraftTools.py` module.
+
+These tools are initialized by `InitGui.py`, and require the graphical
+interface to exist.
+
+Those commands that require a "task panel" call the respective module
+and class in `drafttaskpanels/`.
+

src/Mod/Draft/draftobjects/README.md

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+2020 February
+
+At the moment these object functions aren't used.
+
+When the Draft tools are eventually split into individual modules,
+the code of the object creation functions should be placed here.
+

src/Mod/Draft/drafttaskpanels/README.md

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+2020 February
+
+These files provide the logic behind the task panel of the GuiCommands
+defined in `draftguitools/`.
+
+The task panel graphical interface is properly defined in
+the `Resources/ui/` files, which are made with QtCreator.
+
+There are many commands which aren't defined in `draftguitools/`.
+These are defined in the big `DraftGui.py` module, which needs to be split
+into individual GuiCommands, and each should have its own dedicated
+`.ui` file.
+

src/Mod/Draft/draftviewproviders/README.md

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+2020 February
+
+At the moment these view providers aren't used at all.
+
+When the Draft tools are eventually split into individual modules,
+the code of the view providers should be placed here.
+