Add doc on using ANGLE instead of SwiftShader GL

docs/ANGLE.md

+# ANGLE
+
+## About
+
+[ANGLE](https://chromium.googlesource.com/angle/angle/) is a driver implementation that translates OpenGL ES to native 3D backends depending on the target system. Such backends include D3D on Windows, OpenGL Desktop, and Vulkan.
+
+While SwiftShader initially offered a GLES frontend, this has been deprecated and replaced by a Vulkan frontend. In order to use SwiftShader for GLES, we now recommend using ANGLE on top of SwiftShader Vulkan.
+
+## Why use ANGLE instead of SwiftShader's GLES frontend?
+
+There are a few reasons why SwiftShader's GLES frontend has been deprecated, and that we now recommend using ANGLE:
+
+1. GLES 3.1 support - ANGLE supports GLES 3.1, while SwiftShader supports 3.0.
+
+2. [ANGLE's GLES 3.1](https://www.khronos.org/conformance/adopters/conformant-products/opengles#submission_907) implementation, along with [SwiftShader's Vulkan 1.1](https://www.khronos.org/conformance/adopters/conformant-products#submission_403) implementation, are both Khronos-certified conformant.
+
+3. ANGLE's GLES validation is more complete than SwiftShader's.
+
+4. Dropping support SwiftShader's GLES frontend allows our team to focus our efforts on implementing a solid and conformant Vulkan frontend instead.
+
+## How to use ANGLE with SwiftShader's Vulkan frontend
+
+In order to build ANGLE, you will need to install
+[depot_tools](https://commondatastorage.googleapis.com/chrome-infra-docs/flat/depot_tools/docs/html/depot_tools_tutorial.html#_setting_up).
+You will also need Python 2 installed. Note that ANGLE does not offer a CMake build at this time. Follow the official [dev setup](https://chromium.googlesource.com/angle/angle/+/HEAD/doc/DevSetup.md) to install all required tools, clone ANGLE and its dependencies, and build it. This will look something like:
+
+```
+cd angle
+gn gen out/Release
+autoninja -C out/Release
+```
+
+This will build all ANGLE targets into the `out/Release` directory, including:
+
+* `libEGL.so`
+* `libGLESv2.so`
+* `libvk_swiftshader.so`
+
+As long as your application uses ANGLE's `libEGL` and `libGLESv2`, ANGLE will take care of translating to a default 3D renderer for the current platform.
+
+There are multiple ways to have ANGLE use SwiftShader Vulkan as its backend:
+
+1. Set `ANGLE_DEFAULT_PLATFORM=swiftshader` environment variable. When running your GLES application, ANGLE will use `libvk_swiftshader` as its Vulkan driver.
+
+2. Set both `ANGLE_DEFAULT_PLATFORM=vulkan` and `VK_ICD_FILENAMES=path/to/angle/out/Debug/libvk_swiftshader_icd.json` environment variables. When running your GLES application, ANGLE will use it's Vulkan backend, and the Vulkan Loader will load `libvk_swiftshader` via the Vulkan Loader.
+
+3. Using the `EGL_ANGLE_platform_angle` extension. As described in [this section](https://chromium.googlesource.com/angle/angle/+/HEAD/doc/DevSetup.md#choosing-a-backend) of ANGLE's setup documentation, you can use the `EGL_ANGLE_platform_angle` extension to select the renderer to use at EGL initialization time. Use the `ANGLE_platform_angle_device_type_swiftshader` device type to select SwiftShader specifically (see [documentation here](https://chromium.googlesource.com/angle/angle/+/master/extensions/EGL_ANGLE_platform_angle_device_type_swiftshader.txt)).