added plugin docs

.drone.yml

@@ -0,0 +1,18 @@
+build
+  image: go
+  commands:
+    - go test
+    - go build
+    - go test
+
+plugin:
+  name: Docker
+  desc: Build and Publish images to a Docker registry.
+  type: publish
+  image: plugins/drone-docker
+  labels:
+    - docker
+    - image
+    - container
+
+  

DOCS.md

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+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
+```