Update README.md with ray tracing shader stages filename extensions

[EdoardoLuciani]

While looking for the ray tracing shaders file extensions I had to look inside the source code to find them

glslang/gtests/TestFixture.cpp

Line 49 in 2c7c84c

EShLanguage GetShaderStage(const std::string& stage)

Since the rasterization filename extensions are already listed why not include the ray tracing ones?

[CLAassistant]

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[greg-lunarg]

LGTM. Thanks for the fix!