Add example for runit service

etc/README.md

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+This directory contains contributed setup examples.

etc/linux-runit/README.md

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+This directory contains a configuration for running syncthing under the
+"runit" service manager on Linux. It probably works perfectly fine on
+other platforms also using runit.
+
+ 1. Install runit.
+
+ 2. Edit the `run` file to set the username to run as, the user's home
+    directory and the place where the syncthing binary lives. It is
+    recommended to place it in a directory writeable by the running user
+    so that automatic upgrades work.
+
+ 3. Copy the edited `run` file to `/etc/service/syncthing/run`.
+
+Log output is sent to syslogd.
+

etc/linux-runit/run

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+#!/bin/sh
+
+export USERNAME=jb
+export HOME="/home/$USERNAME"
+export SYNCTHING="$HOME/bin/syncthing"
+
+setuidgid "$USERNAME" "$SYNCTHING" -logflags 0 2>&1 | logger -t syncthing
+