Add documentation for use with Google Cloud Build (#170)

Signed-off-by: Naseem <naseemkullah@gmail.com>

examples/gcp-cloud-build/README.md

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+# Chart testing example with Google Cloud Build
+
+This example shows how to lint and test charts using [Google Cloud Build](https://cloud.google.com/cloud-build/)
+
+Since Google Cloud Build will ignore copying over `.git` by default, you will need to initialize `git` and add a `remote`. This example assumes that there is a pre-existing GKE cluster with `helm` already installed.

examples/gcp-cloud-build/cloudbuild.yaml

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+steps:
+
+- name: 'gcr.io/cloud-builders/git'
+  id: 'git-init'
+  args: ['init']
+  waitFor: ['-']
+
+- name: 'gcr.io/cloud-builders/git'
+  id: 'git-add-remote'
+  args: ['remote', 'add', 'origin', 'git@github.com:github-account/charts-repo.git']
+  waitFor: ['git-init']
+
+- name: 'gcr.io/cloud-builders/kubectl'
+  id: 'cluster-info'
+  args: ['cluster-info']
+  waitFor: ['git-add-remote']
+
+- name: quay.io/helmpack/chart-testing
+  id: 'lint-and-install-charts'
+  entrypoint: 'ct'
+  args: ['lint-and-install']
+  waitFor: ['cluster-info']
+
+options:
+  env:
+  - CLOUDSDK_COMPUTE_ZONE=cluster-location
+  - CLOUDSDK_CONTAINER_CLUSTER=cluster-name
+
+timeout:
+  3600s