Document use of API Layers
These can be fairly easily used through environment variables (at least, once they work properly... some pr's pending in each https://github.com/KhronosGroup/OpenXR-SDK/pull/17 https://github.com/KhronosGroup/OpenXR-SDK/pull/18).
export XR_API_LAYER_PATH=/home/ryan/src/openxr/build/src/api_layers export XR_ENABLE_API_LAYERS=XR_APILAYER_LUNARG_core_validation
The list of layers to enable is colon-separated (at least on Linux). Docs are available but fairly scattered among the various OpenXR repos.