main.py: Add note about using /tmp

I originally wanted to use an "uploads" directory or something, but
it seems we can only write to /tmp on Google App Engine. They really
want you to buy storage or database services! This is memory mapped
so it disappears when you re-deploy.

main.py

@@ -18,6 +18,8 @@ from werkzeug.utils import secure_filename
 app = Flask(__name__)
 app.config["MAX_CONTENT_LENGTH"] = 1024 * 1024
 app.config["UPLOAD_EXTENSIONS"] = [".csv"]
+# the only place we can write to on Google App Engine is /tmp
+# see: https://cloud.google.com/appengine/docs/standard/python3/using-temp-files
 app.config["UPLOAD_PATH"] = "/tmp"