add note about "sudo chmod u+s /bin/ping" to the readme

[digiltd]

Hi, thanks for the plugin.

Worked great after a lot of faffing. I am not if I am the only one, but when I installed Homebridge I followed the guide about setting it up on the Pi. There is a section regarding systemd about a seperate Homebridge user.

So my config, accessories and persist files are in /var/homebridge/config.json

I haven't ever seen anyone refer to this, and there are many Pi users who refer to their configs as being in the default /home/pi/.homebridge/ location.

Changing the permissions as suggested by @oberstmueller in #7 of the ping command using sudo chmod u+s /bin/ping fixed the issue for me.

But now knowing about the section regarding the homebridge user mentioned above. Is it something that can be changed without having to chmod the ping command?