docs(readme): add readme

README.md

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+# Github Action for Kubernetes CLI
+
+This action provides `kubectl` for Github Actions.
+
+## Usage
+
+```hcl
+workflow "build and deploy" {
+  on = "push"
+  resolves = ["verify deployment"]
+}
+
+action "deploy to cluster" {
+  needs = ["todo: build and push containers"]
+  uses = "steebchen/kubectl@master"
+  args = "set image --record deployment/my-app container=$GITHUB_REPOSITORY:$GITHUB_SHA"
+  secrets = ["KUBE_CONFIG_DATA"]
+}
+
+action "verify deployment" {
+  needs = "deploy to cluster"
+  uses = "steebchen/kubectl@master"
+  args = ["rollout status deployment/aws-example-octodex"]
+  secrets = ["KUBE_CONFIG_DATA"]
+}
+```
+
+## Secrets
+
+`KUBE_CONFIG_DATA` – **required**: A base64-encoded kubeconfig file with credentials for Kubernetes to access the cluster. You can get it by running the following command:
+
+```bash
+cat $HOME/.kube/config | base64
+```
+
+**Note**: Do not use kubectl config view as this will hide the certificate-authority-data.