docs: hide range-related headers from flask

This is to prevent flask from returning a partial response for a range request when nginx is supposed to emulate range requests.
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.

README.rst

@@ -65,6 +65,9 @@ The following Nginx configuration snippet is sufficient::
             proxy_pass http://localhost:8337;
             # this statement forces Nginx to emulate "Range" responses
             proxy_force_ranges on;
+            # Hide range-related headers from flask/backend server
+            proxy_hide_header "Accept-Ranges";
+            proxy_set_header "Range" "";
         }
     }