GH-801 Add CloudEvent SDK sample

Resolves #801

spring-cloud-function-samples/function-sample-cloudevent-sdk/README.adoc

@@ -0,0 +1,25 @@
+## Examples of integration with Cloud-Event SDK
+
+### Introduction
+As described in these two blog posts (https://spring.io/blog/2020/12/10/cloud-events-and-spring-part-1 & https://spring.io/blog/2020/12/23/cloud-events-and-spring-part-2),
+Spring natively supports Cloud Event specification so to work with Cloud Events you don't need anything and `function-sample-cloudevent`, `function-sample-cloudevent-rsocket` and `function-sample-cloudevent-stream` examples show exactly how you can benefit from Spring's transparent handling of Cloud Events.
+
+So this example shows integration of Spring with https://github.com/cloudevents/sdk-java[Cloud Event SDK] (not managed by Spring). 
+
+NOTE: This is only to demonstrate the integration capabilities with Cloud Event SDK (regardless of the reason). It is NOT required to work with Cloud Events.
+
+In this example the `echo` function uses `CloudEvent` type from java SDK to receive Cloud Event and echo it back using `CloudEventBuilder`.
+Note that you also need to register `CloudEventMessageConverter`
+
+
+Simply start the `DemoApplication`. Once started access the `echo` function exposed as an HTTP endpoint
+
+```
+curl -v -d '{"value": "Foo"}' \
+    -H'Content-type: application/json' \
+    -H'ce-id: 1' \
+    -H'ce-source: function-sample-cloudevent-sdk' \
+    -H'ce-type: my.application.Foo' \
+    -H'ce-specversion: 1.0' \
+    http://localhost:8080/echo
+```