add a not about unprivileged mode

README.md

@@ -30,6 +30,12 @@ If you want to install fping from source, proceed as follows:
 3. Make fping either setuid, or, if under Linux:
    `sudo setcap cap_net_raw+ep fping`
 
+If you can't run fping as root or can't use the cap_net_raw capability, you can
+also run fping in unprivileged mode. This works on MacOS and also on Linux,
+provided that your GID is included in the range defined in
+`/proc/sys/net/ipv4/ping_group_range`. This is particularly useful for running
+fping in rootless / unprivileged containers.
+
 ## Usage
 
 Have a look at the [fping(8)](doc/fping.pod) manual page for usage help.