Adds Readme to GCP sample app

Also, fixing a bug in the default constructor of the invoker.

Fixes: #478.

spring-cloud-function-samples/function-sample-gcp/README.adoc

@@ -0,0 +1,49 @@
+:branch: master
+
+=== Google Cloud Functions Sample Application
+
+===== Test locally
+
+Run the function:
+
+----
+mvn function:run
+----
+
+Invoke the HTTP function:
+
+----
+curl http://localhost:8080/ -d "hello"
+----
+
+===== Deploy to GCP
+
+As of March 2020, Google Cloud Functions for Java is in Alpha.
+You can get on the https://docs.google.com/forms/d/e/1FAIpQLScC98jGi7CfG0n3UYlj7Xad8XScvZC8-BBOg7Pk3uSZx_2cdQ/viewform[whitelist] to try it out.
+
+Package the application.
+
+----
+mvn package
+----
+
+You should see the fat jar in the `deploy` directory.
+
+Make sure that you have the https://cloud.google.com/sdk/install[Cloud SDK CLI] installed.
+
+Run the following command from the project root to deploy.
+
+----
+gcloud alpha functions deploy function-sample-gcp \
+--entry-point org.springframework.cloud.function.adapter.gcloud.FunctionInvoker \
+--runtime java11 \
+--trigger-http \
+--source deploy \
+--memory 512MB
+----
+
+Invoke the HTTP function:
+
+----
+curl https://REGION-PROJECT_ID.cloudfunctions.net/function-sample-gcp -d "hello"
+----