Added a note about reassigning app

I spent multiple days stuck on this and I would not have known without the example from a different mu-semtech repo

Author: Denperidge

README.md

@@ -183,3 +183,7 @@ Since this template is based on the meinheld-gunicorn-docker image, all possible
 ### Production
 
 For hosting the app in a production setting, the template depends on [meinheld-gunicorn-docker](https://github.com/tiangolo/meinheld-gunicorn-docker). All [environment variables](https://github.com/tiangolo/meinheld-gunicorn-docker#environment-variables) used by meinheld-gunicorn can be used to configure your service as well.
+
+## Other
+### Reassigning `app`
+In regular Flask applications (e.g. those not run within this template) you are required to define `app` by using `app = Flask(__name__)` or similar. This does *not* need to be done in your web.py, as this is handled by the microservice architecture/template. Redefining this may cause `The requested URL was not found on the server. If you entered the URL manually please check your spelling and try again.` to be thrown on your routes, which can be luckily be fixed by simply removing the previously mentioned `app = ...` line.