readme: update instuctions for sudo

On CentOS, sudoers sets "requiretty" by default. Even if the procket
helper is run as root, sudo will fail and procket will return
{error,eperm}:

    # id
    uid=0(root) gid=0(root) groups=0(root)

    # erl -pa ebin
    1> {error,eperm} = procket:open(34, [{protocol,tcp}, {family,inet},
            {type,stream}]).

Add instructions for disabling "requiretty" by program or by user.

Thanks to @josemic and xingang shi!