document proxy setup in guide/secure.md (for #26)

The guide is not as quick to follow and amateur-friendly as I'd like. A
few things that might improve matters:

   * complete #27 (built-in https+letsencrypt), so that when not sharing
     the port, users don't need to use nginx or certbot.
   * more ubiquitous IPv6 (out of my control but should happen over
     time) to reduce need to share the port
   * embed a dynamic DNS client
   * support UPnP Internet Gateway Device Control Protocol (if common
     routers have this enabled? probably not for security reasons.)

It's progress, though. Enough that I think I'll merge the auth branch
into master shortly.

scripts/install.sh

@@ -106,7 +106,8 @@ After=network-online.target
 ExecStart=${SERVICE_BIN} run \\
     --db-dir=${DB_DIR} \\
     --ui-dir=${LIB_DIR}/ui \\
-    --http-addr=0.0.0.0:${NVR_PORT}
+    --http-addr=0.0.0.0:${NVR_PORT} \
+    --require=auth=false
 Environment=TZ=:/etc/localtime
 Environment=MOONFIRE_FORMAT=google-systemd
 Environment=MOONFIRE_LOG=info