Read randomness from /dev/urandom rather than /dev/random
Read randomness from /dev/urandom rather than /dev/random

I accidentally changed this in 606a1f1b93b1db25041d4fa6f3b07761356a175d
so this is the corresponding documentation change.

Fixes #4

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@eu.citrix.com>

mirage/xentropyd