Added caveats

This is a old problem which affects sshuttle but there is little information about it and it can be very hard to track down so I think it's important to notify people about it.

Reference to somebody else having the problem already 2012 https://groups.google.com/d/msg/sshuttle/SqHtn1L0_PE/G4bsZvLo17AJ

The reference to it breaking other things comes from the fact that it apparently (I did not test) breaks (or broke) the Junos Pulse VPN client https://groups.google.com/forum/#!searchin/sshuttle/net.inet.ip.scopedroute/sshuttle/KqRbUkLRe5E/2rMRPr208WIJ
Other than the mailing list referred to, I don't think there is a issue tracking system for the sshuttle project, at least I found none.