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.

guide/install-manual.md

@@ -84,7 +84,8 @@ Moonfire NVR can be run as a systemd service. Create
     [Service]
     ExecStart=/usr/local/bin/moonfire-nvr run \
         --db-dir=/var/lib/moonfire-nvr/db \
-        --http-addr=0.0.0.0:8080
+        --http-addr=0.0.0.0:8080 \
+        --require-auth=false
     Environment=TZ=:/etc/localtime
     Environment=MOONFIRE_FORMAT=google-systemd
     Environment=MOONFIRE_LOG=info