Update docs and launchd configuration - stdout not used

[nirs]

Since #58 we log only to stderr, so /var/log/socket_vmnet/stdout is never used.

• Remove from README.md
• Remove from launchd configuration

[AkihiroSuda]

We may still accidentally have some output in stdout, so I suggest keeping the stdout log too.

[nirs]

Ok, maybe just add a node that we log to stderr and stdout is unused.