docs: add plugin usage docs (#32)

DOCS.md

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+## Description
+
+This plugin enables the ability to manage resources in [Kubernetes](https://kubernetes.io/) in a Vela pipeline.
+
+Source Code: https://github.com/go-vela/vela-kubernetes
+
+Registry: https://hub.docker.com/r/target/vela-kubernetes
+
+## Usage
+
+Sample of applying Kubernetes files:
+
+```yaml
+steps:
+  - name: kubernetes
+    image: target/vela-kubernetes:v0.1.0
+    pull: true
+    parameters:
+      action: apply
+      files: [ kubernetes/common, kubernetes/dev/deploy.yml ]
+```
+
+Sample of pretending to apply Kubernetes files:
+
+```diff
+steps:
+  - name: kubernetes
+    image: target/vela-kubernetes:v0.1.0
+    pull: true
+    parameters:
+      action: apply
++     dry_run: true
+      files: [ kubernetes/common, kubernetes/dev/deploy.yml ]
+```
+
+Sample of patching containers in Kubernetes files:
+
+```yaml
+steps:
+  - name: kubernetes
+    image: target/vela-kubernetes:v0.1.0
+    pull: true
+    parameters:
+      action: patch
+      files: [ kubernetes/common, kubernetes/dev/deploy.yml ]
+      containers:
+        - name: sample
+          image: alpine:latest
+```
+
+Sample of pretending to patch containers in Kubernetes files:
+
+```diff
+steps:
+  - name: kubernetes
+    image: target/vela-kubernetes:v0.1.0
+    pull: true
+    parameters:
+      action: patch
++     dry_run: true
+      files: [ kubernetes/common, kubernetes/dev/deploy.yml ]
+      containers:
+        - name: sample
+          image: alpine:latest
+```
+
+Sample of watching the status of resources:
+
+```yaml
+steps:
+  - name: kubernetes
+    image: target/vela-kubernetes:v0.1.0
+    pull: true
+    parameters:
+      action: status
+      statuses: [ sample ]
+```
+
+## Secrets
+
+**NOTE: Users should refrain from configuring sensitive information in your pipeline in plain text.**
+
+You can use Vela secrets to substitute sensitive values at runtime:
+
+```diff
+steps:
+  - name: kubernetes
+    image: target/vela-kubernetes:v0.1.0
+    pull: true
++   secrets: [ kube_config ]
+    parameters:
+      action: apply
+      files: [ kubernetes/common, kubernetes/dev/deploy.yml ]
+-     config: |
+-     ---
+-     apiVersion: v1
+-     kind: Config
+```
+
+## Parameters
+
+The following parameters are used to configure the image:
+
+| Name        | Description                                          | Required | Default |
+| ----------- | ---------------------------------------------------- | -------- | ------- |
+| `action`    | action to perform against Kubernetes                 | `true`   | `N/A`   |
+| `cluster`   | Kubernetes cluster from the configuration file       | `false`  | `N/A`   |
+| `context`   | Kubernetes context from the configuration file       | `false`  | `N/A`   |
+| `file`      | configuration file for communication with Kubernetes | `true`   | `N/A`   |
+| `log_level` | set the log level for the plugin                     | `true`   | `info`  |
+| `namespace` | Kubernetes namespace from the configuration file     | `false`  | `N/A`   |
+
+#### Apply
+
+The following parameters are used to configure the `apply` action:
+
+| Name      | Description                                      | Required | Default |
+| --------- | ------------------------------------------------ | -------- | ------- |
+| `dry_run` | enables pretending to perform the apply          | `false`  | `false` |
+| `files`   | list of Kubernetes files or directories to apply | `true`   | `false` |
+| `output`  | set the output for the apply                     | `false`  | `false` |
+
+#### Patch
+
+The following parameters are used to configure the `patch` action:
+
+| Name         | Description                                      | Required | Default |
+| ------------ | ------------------------------------------------ | -------- | ------- |
+| `containers` | containers from the files to patch               | `true`   | `false` |
+| `dry_run`    | enables pretending to perform the patch          | `false`  | `false` |
+| `files`      | list of Kubernetes files or directories to patch | `true`   | `false` |
+| `output`     | set the output for the patch                     | `false`  | `false` |
+
+#### Status
+
+The following parameters are used to configure the `status` action:
+
+| Name       | Description                                      | Required | Default |
+| ---------- | ------------------------------------------------ | -------- | ------- |
+| `statuses` | list of Kubernetes resources to watch status on  | `true`   | `N/A`   |
+| `timeout`  | total time allowed to watch Kubernetes resources | `true`   | `5m`    |
+| `watch`    | enables watching until the resource completes    | `false`  | `true`  |
+
+## Template
+
+COMING SOON!
+
+## Troubleshooting
+
+Below are a list of common problems and how to solve them: