tools/wltests: README: add initial documentation
tools/wltests: README: add initial documentation

Mainly a placeholder... to be further extended.

Signed-off-by: Patrick Bellasi <431c86f368affbc3148c4669fbd465131f7e82cb@arm.com>

credp/lisa,credp/lisa,ARM-software/lisa,bjackman/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,bjackman/lisa,ARM-software/lisa,credp/lisa