Update README to include note about arping version

[wtip]

I'm using Ubuntu 16.04 servers

The mha-helper was failing when running the arping command with requires_arping configured.
I had run apt install arping
I tried manually running the arping command without the -q quiet flag and was getting timeout messages.
I discovered this thread https://bugs.launchpad.net/neutron/+bug/1715734 and installed iputils-arping instead which fixed the error code.
I also had to do a ln -s /usr/bin/arping /usr/sbin/arping because this arping version installs at a different path.

[wtip]

In case anyone is wondering why arping always fails on Ubuntu 20.04 Focal it's because a bug was introduced that always returns a exit code 1.
This was fixed with this commit iputils/iputils@68f12fc but that version is not available in the Focal repo at this time

My quick hack to fix this is to modify this line

mha-helper/mha_helper/vip_metal_helper.py

Line 40 in fd2799b

arping_cmd = "%s -q -c 3 -A -I %s %s" % (VIPMetalHelper.ARPING_CMD, self._cluster_interface, self._writer_vip)

to this
arping_cmd = "%s -q -c 3 -A -I %s %s || true" % (VIPMetalHelper.ARPING_CMD, self._cluster_interface, self._writer_vip)

So that the exit code is always 0.