Add SDK readme.

Author: rpavlik

github/sdk/README.md

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+# OpenXR™ Software Development Kit (SDK) Project
+
+This repository contains OpenXR headers, as well as source code and build scripts
+for the OpenXR loader.
+It contains all generated source files and headers pre-generated for minimum dependencies.
+
+The authoritative public repository for this project is located at
+<https://github.com/KhronosGroup/OpenXR-SDK>.
+
+The public repository containing the scripts that generate the files in this repository
+is located at
+<https://github.com/KhronosGroup/OpenXR-SDK-Source>.
+It hosts the public Issue tracker, and accepts patches (Pull Requests) from the
+general public.
+That repository is also where sample code (hello_xr) and API layer source can be found.
+
+**Note that this repo is effectively read-only: changes to this repo should be made in the [OpenXR-SDK-Source](https://github.com/KhronosGroup/OpenXR-SDK-Source) repo instead**
+
+## Directory Structure
+
+- `BUILDING.md` - Instructions for building the projects
+- `README.md` - This file
+- `COPYING.md` - Copyright and licensing information
+- `CODE_OF_CONDUCT.md` - Code of Conduct
+- `external/` - External code for projects in the repo
+- `include/` - OpenXR header files
+- `src/external/jsoncpp` - The jsoncpp project source code, an included dependency of the loader.
+- `src/loader` - OpenXR loader code, **including generated code**
+
+## Building
+
+The project is set up to build using CMake.
+
+### (Optional) Building the OpenXR Loader as a DLL
+
+By default, the OpenXR loader is built as a static library on Windows and a dynamic library on other platforms.
+To specify alternate behavior, define the CMake option `DYNAMIC_LOADER`,
+e.g. by adding `-DDYNAMIC_LOADER=ON` or `-DDYNAMIC_LOADER=OFF` to your CMake command line.
+
+### Windows
+
+Building the OpenXR components in this tree on Windows is supported using
+Visual Studio 2013 and newer.  Before beginning, make sure the appropriate
+"msbuild.exe" is in your PATH.  Also, when generating the solutions/projects
+using CMake, be sure to use the correct compiler version number.  The
+following table is provided to help you:
+
+| Visual Studio        | Version Number |
+| -------------------- |:--------------:|
+| Visual Studio 2013   |       12       |
+| Visual Studio 2015   |       14       |
+| Visual Studio 2017   |       15       |
+
+Specific sample command lines for building follow.
+If you're already familiar with the process of building a project with
+CMake, you may skim or skip these instructions.
+
+#### Windows 64-bit
+
+First, generate the 64-bit solution and project files using CMake:
+
+```cmd
+mkdir build\win64
+cd build\win64
+cmake -G "Visual Studio [Version Number] Win64" ..\..
+```
+
+Finally, open the `build\win64\OPENXR.sln` in the Visual Studio to build the loader.
+
+#### Windows 32-bit
+
+First, generate the 32-bit solution and project files using CMake:
+
+```cmd
+mkdir build\win32
+cd build\win32
+cmake -G "Visual Studio [Version Number]" ..\..
+```
+
+Open the `build\win32\OPENXR.sln` in the Visual Studio to build the loader.
+
+### Linux
+
+The following set of Debian/Ubuntu packages provides all required libs for building for xlib or xcb with OpenGL and Vulkan support.
+
+- `build-essential`
+- `cmake` (of _somewhat_ recent vintage, 3.10+ known working)
+- `libgl1-mesa-dev`
+- `libvulkan-dev`
+- `libx11-xcb-dev`
+- `libxcb-dri2-0-dev`
+- `libxcb-glx0-dev`
+- `libxcb-icccm4-dev`
+- `libxcb-keysyms1-dev`
+- `libxcb-randr0-dev`
+- `libxrandr-dev`
+- `libxxf86vm-dev`
+- `mesa-common-dev`
+
+Specific sample command lines for building follow.
+If you're already familiar with the process of building a project with
+CMake, you may skim or skip these instructions.
+
+#### Linux Debug
+
+```sh
+mkdir -p build/linux_debug
+cd build/linux_debug
+cmake -DCMAKE_BUILD_TYPE=Debug ../..
+make
+```
+
+#### Linux Release
+
+```sh
+mkdir -p build/linux_release
+cd build/linux_release
+cmake -DCMAKE_BUILD_TYPE=Release ../..
+make
+```