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 https://github.com/iputils/iputils/commit/68f12fc4a0dbef4ae4c404da24040d22c5a14339 but that version is not available in the Focal repo at this time

My quick hack to fix this is to modify this line https://github.com/ovaistariq/mha-helper/blob/fd2799b6f70b31e91805857d40abbef39e7db334/mha_helper/vip_metal_helper.py#L40 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.