Add natvis schema and a quick page on natvis (#1400)

* Add natvis schema and a quick page on natvis

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Author: pieandcakes

Documentation/Debugger/natvis/Natvis.md

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+# Natvis - Custom views for native objects
+
+Natvis framework is a framework that allows developers to write custom schemas to help visualize native objects. 
+
+For gdb/lldb debugging (`"type": "cppdbg"`), a subset of the Natvis framework has been ported to the C/C++ extension and the code resides in the [MIEngine](https://github.com/Microsoft/MIEngine) shared component. If additional features that are not implemented are requested, please file an [issue](https://github.com/Microsoft/MIEngine/issues) on the MIEngine GitHub page with details of what is missing.
+
+For Visual C++ debugging (`"type": "cppvsdbg"`), the debugger contains the full implementation of the Natvis framework as Visual Studio. 
+
+## Documentation
+
+Official documentation can be found [here](https://docs.microsoft.com/en-us/visualstudio/debugger/create-custom-views-of-native-objects).
+
+## Schema
+
+The natvis schema can be found [here](natvis.xsd).