Deploying hugo-mx-gateway on Google App Engine

This requires to have an active GCP account and Google Cloud SDK (gcloud) installed on your work station.

Create/select a GCP project to deploy hugo-mx-gateway.

Note that each GCP project can hold only a single App Engine instance. Several applications can be co-hosted as services for the root App Engine instance. In this case, a new application has to be declared as service in the app.yaml file.
Create the Google App Engine configuration file
```
cp app.yaml.sample app.yaml
```
Open the app.yaml file with your favorite editor.
Edit the configuration variables as described here.
Start the deployment
```
make deploy-gcp
```
Check that hugo-mx-gateway is up and working
```
curl https://PROJECT-ID.REGION.r.appspot.com/ 
```
Replace PROJECT-ID with the GCP project ID, and REGION with the deployment region.*

The output in case of success shall be {"status": "ok"}.