Document EGL vendor ICD search paths

Created by: smcv

The behaviour of a relative library_path is deliberately left unspecified here. As an implementation detail, it's currently passed directly to dlopen(), so in practice it will be interpreted as relative to the process's current working directory, but that doesn't seem practically useful. Vulkan treats relative paths as being relative to the JSON file in which they were found, which seems more likely to be a helpful behaviour.

Otherwise this documents the current behaviour, without any of the possible changes suggested on #187 (closed). If any of those changes are made, then this document should be updated at the same time.