Hello world - Go for Knative Eventing. (#1862)

* App + readme

* vendored packages - cloudevents/sdk-go and its dependencies

* Mark-ups updated. Final testing on a new cluster pending.

* final changes

Author: akashrv

Gopkg.lock

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+Following examples include a simple web app written in the language of your choice that you can use to test knative eventing. It shows how to consume a [CloudEvent](https://cloudevents.io/) in Knative eventing, and optionally how to respond back with another CloudEvent in the http response.
+
+We will deploy the app as a [Kubernetes Deployment](https://kubernetes.io/docs/concepts/workloads/controllers/deployment/) along with a [Kubernetes Service](https://kubernetes.io/docs/concepts/services-networking/service/). However, you can also deploy the app as a [Knative Serving Service](../../../../serving/README.md)
+
+## Prerequisites
+
+- A Kubernetes cluster with [Knative Eventing](../../getting-started.md#installing-knative-eventing) installed.
+  - If you decide to deploy the app as a Knative Serving Service then you will have to install [Knative Serving](../../../install/README.md).
+- [Docker](https://www.docker.com) installed and running on your local machine,
+  and a Docker Hub account configured (we'll use it for a container registry).

docs/eventing/samples/helloworld/README.md

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+---
+title: "Knative Eventing - Hello World app"
+linkTitle: "Hello World"
+weight: 10
+type: "docs"
+---
+
+{{% readfile file="README.md" relative="true" markdown="true" %}}

docs/eventing/samples/helloworld/_index.md

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+# Use the offical Golang image to create a build artifact.
+# This is based on Debian and sets the GOPATH to /go.
+# https://hub.docker.com/_/golang
+FROM golang:1.12 as builder
+
+# Copy local code to the container image.
+WORKDIR /go/src/github.com/knative/docs/helloworld
+COPY . .
+
+# Build the command inside the container.
+# (You may fetch or manage dependencies here,
+# either manually or with a tool like "godep".)
+RUN curl -fsSL -o /usr/local/bin/dep https://github.com/golang/dep/releases/download/v0.5.4/dep-linux-amd64 && chmod +x /usr/local/bin/dep
+
+RUN dep init
+RUN dep ensure -vendor-only
+
+
+RUN CGO_ENABLED=0 GOOS=linux go build -v -o helloworld
+
+# Use a Docker multi-stage build to create a lean production image.
+# https://docs.docker.com/develop/develop-images/multistage-build/#use-multi-stage-builds
+FROM alpine
+RUN apk add --no-cache ca-certificates
+
+# Copy the binary to the production image from the builder stage.
+COPY --from=builder /go/src/github.com/knative/docs/helloworld/helloworld /helloworld
+
+# Run the web service on container startup.
+CMD ["/helloworld"]