Added a comment to tor.service.in

This explains that if you change your torrc to do more, you might
need to change tor.service.in to allow it.  See #15195.

contrib/dist/tor.service.in

@@ -1,3 +1,9 @@
+# tor.service -- this systemd configuration file for Tor sets up a
+# relatively conservative, hardened Tor service.  You may need to
+# edit it if you are making changes to your Tor configuration that it
+# does not allow.  Package maintainers: this should be a starting point
+# for your tor.service; it is not the last point.
+
 [Unit]
 Description = Anonymizing overlay network for TCP
 After = syslog.target network.target nss-lookup.target