added plugin docs

DOCS.md

@@ -0,0 +1,21 @@
+Use the Docker plugin to build and push Docker images to a registry.
+The following parameters are used to configuration this plugin:
+
+* **username** - authenticates with this username
+* **password** - authenticates with this password
+* **email** - authenticates with this email
+* **repo** - repository name for the image
+* **tag** - repository tag for the image
+
+The following is a sample Docker configuration in your .drone.yml file:
+
+```yaml
+pubish:
+  docker:
+    username: kevinbacon
+    password: $$DOCKER_PASSWORD
+    email: kevin.bacon@mail.com
+    repo: foo/bar
+    tag: latest
+    file: Dockerfile
+```