Add note about use of /dev/random (WOCommon, 13b9765)

Reading too many bits from /dev/random may constitute an abuse of a scarce resource, and the read() call may block if there is not enough random data anyway. Ideally in this case we should only read enough to seed another (psuedo-random) generator.

Signed-off-by: Greg Hurrell <greg@hurrell.net>