Mention the CR vs. LF problem in the documentation

Let's hope that this will help to avoid that people run into the same
problem again in the future. At least we've got some lines of documentation
this way where we can point people to who try to use "nc" in the wrong way.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>