First draft of a useful README [#8]

README.md

@@ -1,3 +1,71 @@
 # Drone plugin for Helm 3
 
-Dissatisfied with this empty README? Consider grabbing [the "put stuff in the README" issue](https://github.com/pelotech/drone-helm3/issues/8)!
+This plugin provides an interface between [Drone](https://drone.io/) and [Helm 3](https://github.com/kubernetes/helm):
+
+* Lint your charts
+* Deploy your service
+* Delete your service
+
+The plugin is inpsired by [drone-helm](https://github.com/ipedrazas/drone-helm), which fills the same role for Helm 2. It provides a comparable feature-set and the configuration settings are backwards-compatible.
+
+## Example configuration
+
+These examples give a minimal and sufficient configuration for each use-case. For a full description of each case, see [docs/lint_example.yml](docs/lint_example.yml), [docs/upgrade_example.yml](docs/upgrade_example.yml), and [docs/delete_example.yml](docs/delete_example.yml).
+
+### Linting
+
+```yaml
+steps:
+  - name: lint
+    image: pelotech/drone-helm3
+    settings:
+      helm_command: lint
+      chart: ./
+```
+
+### Deployment
+
+```yaml
+steps:
+  - name: deploy
+    image: pelotech/drone-helm3
+    settings:
+      helm_command: upgrade
+      chart: ./
+      release: my-project
+    environment:
+      API_SERVER: https://my.kubernetes.installation/clusters/a-1234
+      KUBERNETES_TOKEN:
+        from_secret: kubernetes_token
+```
+
+### Deletion
+
+```yaml
+steps:
+  - name: delete
+    image: pelotech/drone-helm3
+    settings:
+      helm_command: delete
+      release: my-project
+    environment:
+      API_SERVER: https://my.kubernetes.installation/clusters/a-1234
+      KUBERNETES_TOKEN:
+        from_secret: kubernetes_token
+```
+
+## Upgrading from drone-helm
+
+The setting names for drone-helm3 are backwards-compatible with those for drone-helm, so the only mandatory step is to update the `image` clause so that drone uses the new plugin.
+
+There are several settings that no longer have any effect:
+
+* `purge` -- this is the default behavior in Helm 3
+* `recreate_pods`
+* `tiller_ns`
+* `upgrade`
+* `canary_image`
+* `client_only`
+* `stable_repo_url`
+
+If your `.drone.yml` contains those settings, we recommend removing them.