docs: support range requests w. gstreamer/firefox

Playing large opus files within firefox and gstreamer/mopidy worked for me only after I made this change in my nginx config.
I guess this was not a problem previously since older flask versions did not support range queries at all but that seems to have changed partially since flask returns an `Accept-Ranges: bytes` header.
Though, when using flask for streaming directly (without reverse-proxy), audio still doesn't play within firefox or gstreamer.
gstreamer logged an error indicating the request is unsupported.
Both gstreamer and firefox started working only after I disabled passing through the range headers.

README.rst

@@ -65,6 +65,10 @@ The following Nginx configuration snippet is sufficient::
             proxy_pass http://localhost:8337;
             # this statement forces Nginx to emulate "Range" responses
             proxy_force_ranges on;
+            # Prevent flask server from handling range requests.
+            # (flask does not fully support range requests)
+            proxy_hide_header "Accept-Ranges";
+            proxy_set_header "Range" "";
         }
     }