Document EGL vendor ICD search paths

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. If any of those changes are made,
then this document should be updated at the same time.

Signed-off-by: Simon McVittie <smcv@collabora.com>