Add info about reliance on avahi-publish-service

For people who need to use `--with-external-mdns` (particularly installing in a container on a host where avahi already exists), they should ensure they have avahi-publish-service available.

diff --git a/README.md b/README.md
index 0efe90aaf78ca34524686626bd640237b0fc20bf..2c32bbecd9a39008d8fc31a6fdc1187fb82d3cd9 100644 (file)
--- a/README.md
+++ b/README.md
@@ -191,7 +191,7 @@ $ autoreconf -i -f
 - `--with-stdout` include an optional backend module to enable raw audio to be output through standard output (stdout).
 - `--with-pipe` include an optional backend module to enable raw audio to be output through a unix pipe.
 - `--with-soundio` include an optional backend module to enable raw audio to be output through the soundio system.
-- `--with-avahi` or `--with-tinysvcmdns` for mdns support. Avahi is a widely-used system-wide zero-configuration networking (zeroconf) service — it may already be in your system. If you don't have Avahi, or similar, then consider including tinysvcmdns, which is a tiny zeroconf service embedded inside the shairport-sync application itself. To enable multicast for `tinysvcmdns`, you may have to add a default route with the following command: `route add -net 224.0.0.0 netmask 224.0.0.0 eth0` (substitute the correct network port for `eth0`). You should not have more than one zeroconf service on the same system — bad things may happen, according to RFC 6762, §15.
+- `--with-avahi` or `--with-tinysvcmdns` for mdns support. Avahi is a widely-used system-wide zero-configuration networking (zeroconf) service — it may already be in your system. If you don't have Avahi, or similar, then consider including tinysvcmdns, which is a tiny zeroconf service embedded inside the shairport-sync application itself. To enable multicast for `tinysvcmdns`, you may have to add a default route with the following command: `route add -net 224.0.0.0 netmask 224.0.0.0 eth0` (substitute the correct network port for `eth0`). You should not have more than one zeroconf service on the same system — bad things may happen, according to RFC 6762, §15. If you need to use an external zeroconf service (`--with-external-mdns`) to avoid this problem, you may need to install avahi-utils or avahi-tools to get the `avahi-publish-service` tool for your system.
 - `--with-ssl=openssl`, `--with-ssl=mbedtls` or `--with-ssl=polarssl` (deprecated) for encryption and related utilities using either OpenSSL, mbed TLS or PolarSSL.
 - `--with-libdaemon` include a demonising library needed if you want to be able to demonise Shairport Sync with the `-d` option. Not needed for `systemd`-based systems which demonise programs differently.
 - `--with-soxr` for libsoxr-based resampling.