Add docs / samples for Sequence

docs/eventing/samples/sequence/sequence-reply-to-event-display/README.md

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+---
+title: "Sequence Wired to event-display"
+weight: 20
+type: "docs"
+---
+
+# Using Sequences in series
+
+## Overview
+
+We are going to create the following logical configuration. We create a CronJobSource,
+feeding events to a (`Sequence`)[../../../sequence.md], then taking the output of that `Sequence` and
+displaying the resulting output.
+
+![Logical Configuration](./sequence-reply-to-event-display.png)
+
+## Prerequisites
+
+For this example, we'll assume you have set up an `InMemoryChannel`
+as well as Knative Serving (for our functions). The examples use `default`
+namespace, again, if you want to deploy to another Namespace, you will need
+to modify the examples to reflect this.
+
+If you want to use different type of `Channel`, you will have to modify the
+`Sequence.Spec.ChannelTemplate` to create the appropriate Channel resources.
+
+
+## Setup
+
+### Create the Knative Services
+
+Change `default` below to create the steps in the Namespace where you want resources
+created.
+
+```yaml
+apiVersion: serving.knative.dev/v1beta1
+kind: Service
+metadata:
+  name: first
+spec:
+  template:
+    spec:
+      containers:
+      - image: us.gcr.io/probable-summer-223122/cmd-03315b715ae8f3e08e3a9378df706fbb@sha256:2656f39a7fcb6afd9fc79e7a4e215d14d651dc674f38020d1d18c6f04b220700
+        env:
+        - name: STEP
+          value: "0"
+
+---
+apiVersion: serving.knative.dev/v1beta1
+kind: Service
+metadata:
+  name: second
+spec:
+  template:
+    spec:
+      containers:
+      - image: us.gcr.io/probable-summer-223122/cmd-03315b715ae8f3e08e3a9378df706fbb@sha256:2656f39a7fcb6afd9fc79e7a4e215d14d651dc674f38020d1d18c6f04b220700
+        env:
+        - name: STEP
+          value: "1"
+---
+apiVersion: serving.knative.dev/v1beta1
+kind: Service
+metadata:
+  name: third
+spec:
+  template:
+    spec:
+      containers:
+      - image: us.gcr.io/probable-summer-223122/cmd-03315b715ae8f3e08e3a9378df706fbb@sha256:2656f39a7fcb6afd9fc79e7a4e215d14d651dc674f38020d1d18c6f04b220700
+        env:
+        - name: STEP
+          value: "2"
+---
+```
+
+
+```shell
+kubectl -n default create -f ./steps.yaml
+```
+
+### Create the Sequence
+
+The `sequence.yaml` file contains the specifications for creating the Sequence. If you are using a different type of Channel, you need to change the
+spec.channelTemplate to point to your desired Channel.
+
+```yaml
+apiVersion: messaging.knative.dev/v1alpha1
+kind: Sequence
+metadata:
+  name: sequence
+spec:
+  channelTemplate:
+    apiVersion: messaging.knative.dev/v1alpha1
+    kind: InMemoryChannel
+  steps:
+  - ref:
+      apiVersion: serving.knative.dev/v1beta1
+      kind: Service
+      name: first
+  - ref:
+      apiVersion: serving.knative.dev/v1beta1
+      kind: Service
+      name: second
+  - ref:
+      apiVersion: serving.knative.dev/v1beta1
+      kind: Service
+      name: third
+  reply:
+    kind: Service
+    apiVersion: serving.knative.dev/v1beta1
+    name: event-display
+```
+
+Change `default` below to create the `Sequence` in the Namespace where you want the
+resources to be created.
+```shell
+kubectl -n default create -f ./sequence.yaml
+```
+
+
+### Create the Service displaying the events created by Sequence
+
+```yaml
+apiVersion: serving.knative.dev/v1beta1
+kind: Service
+metadata:
+  name: event-display
+spec:
+  template:
+    spec:
+      containers:
+      - image: gcr.io/knative-releases/github.com/knative/eventing-sources/cmd/event_display
+```
+
+Change `default` below to create the `Sequence` in the Namespace where you want your resources
+to be created.
+
+```shell
+kubectl -n default create -f ./event-display.yaml
+```
+
+### Create the CronJobSource targeting the Sequence
+
+This will create a CronJobSource which will send a CloudEvent with {"message": "Hello world!"} as
+the data payload every 2 minutes.
+
+```yaml
+apiVersion: sources.eventing.knative.dev/v1alpha1
+kind: CronJobSource
+metadata:
+  name: cronjob-source
+spec:
+  schedule: "*/2 * * * *"
+  data: '{"message": "Hello world!"}'
+  sink:
+    apiVersion: messaging.knative.dev/v1alpha1
+    kind: Sequence
+    name: sequence
+```
+
+```shell
+kubectl -n default create -f ./cron-source.yaml
+```
+
+### Inspecting the results
+
+You can now see the final output by inspecting the logs of the event-display pods.
+
+```shell
+kubectl -n default get pods
+```
+
+Then look at the logs for the event-display pod:
+
+
+```shell
+kubectl -n default logs -l serving.knative.dev/service=event-display -c user-container
+☁️  cloudevents.Event
+Validation: valid
+Context Attributes,
+  cloudEventsVersion: 0.1
+  eventType: samples.http.mod3
+  source: /transformer/2
+  eventID: df52b47e-02fd-45b2-8180-dabb572573f5
+  eventTime: 2019-06-18T14:18:42.478140635Z
+  contentType: application/json
+Data,
+  {
+    "id": 0,
+    "message": "Hello world! - Handled by 0 - Handled by 1 - Handled by 2"
+  }
+```
+
+And you can see that the initial Cron Source message ("Hello World!") has been appended to it by each
+of the steps in the Sequence.

docs/eventing/samples/sequence/sequence-reply-to-event-display/cron-source.yaml

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+apiVersion: sources.eventing.knative.dev/v1alpha1
+kind: CronJobSource
+metadata:
+  name: cronjob-source
+spec:
+  schedule: "*/2 * * * *"
+  data: '{"message": "Hello world!"}'
+  sink:
+    apiVersion: messaging.knative.dev/v1alpha1
+    kind: Sequence
+    name: sequence

docs/eventing/samples/sequence/sequence-reply-to-event-display/event-display.yaml

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+apiVersion: serving.knative.dev/v1beta1
+kind: Service
+metadata:
+  name: event-display
+spec:
+  template:
+    spec:
+      containers:
+      - image: gcr.io/knative-releases/github.com/knative/eventing-sources/cmd/event_display

docs/eventing/samples/sequence/sequence-reply-to-event-display/sequence.yaml

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+apiVersion: messaging.knative.dev/v1alpha1
+kind: Sequence
+metadata:
+  name: sequence
+spec:
+  channelTemplate:
+    apiVersion: messaging.knative.dev/v1alpha1
+    kind: InMemoryChannel
+  steps:
+  - ref:
+      apiVersion: serving.knative.dev/v1beta1
+      kind: Service
+      name: first
+  - ref:
+      apiVersion: serving.knative.dev/v1beta1
+      kind: Service
+      name: second
+  - ref:
+      apiVersion: serving.knative.dev/v1beta1
+      kind: Service
+      name: third
+  reply:
+    kind: Service
+    apiVersion: serving.knative.dev/v1beta1
+    name: event-display

docs/eventing/samples/sequence/sequence-reply-to-event-display/steps.yaml

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+apiVersion: serving.knative.dev/v1alpha1
+kind: Service
+metadata:
+  name: first
+spec:
+  template:
+    spec:
+      containers:
+      - image: us.gcr.io/probable-summer-223122/cmd-03315b715ae8f3e08e3a9378df706fbb@sha256:2656f39a7fcb6afd9fc79e7a4e215d14d651dc674f38020d1d18c6f04b220700
+        env:
+        - name: STEP
+          value: "0"
+
+---
+apiVersion: serving.knative.dev/v1alpha1
+kind: Service
+metadata:
+  name: second
+spec:
+  template:
+    spec:
+      containers:
+      - image: us.gcr.io/probable-summer-223122/cmd-03315b715ae8f3e08e3a9378df706fbb@sha256:2656f39a7fcb6afd9fc79e7a4e215d14d651dc674f38020d1d18c6f04b220700
+        env:
+        - name: STEP
+          value: "1"
+---
+apiVersion: serving.knative.dev/v1alpha1
+kind: Service
+metadata:
+  name: third
+spec:
+  template:
+    spec:
+      containers:
+      - image: us.gcr.io/probable-summer-223122/cmd-03315b715ae8f3e08e3a9378df706fbb@sha256:2656f39a7fcb6afd9fc79e7a4e215d14d651dc674f38020d1d18c6f04b220700
+        env:
+        - name: STEP
+          value: "2"
+---