missing flask app for google app engine

## 🚀 Feature
Some files related to deploying yolov5 on app engine were added in https://github.com/ultralytics/yolov5/pull/964 but still missing is the actual flask app which would comprise of a `main.py`. Additionally a README would be nice here. 

## Motivation
I assume once everything is in place it is a one line command to deploy the app, which would be neat

CC @Guilhen
