(maint) add secrets docs

Author: TP Honey

README.md

@@ -62,6 +62,46 @@ docker build \
 
 > Notice: Be aware that the Docker plugin currently requires privileged capabilities, otherwise the integrated Docker daemon is not able to start.
 
+### Using Docker buildkit Secrets
+
+```yaml
+kind: pipeline
+name: default
+
+steps:
+- name: build dummy docker file and publish
+  image: plugins/docker
+  pull: never
+  settings:
+    repo: tphoney/test
+    tags: latest
+    secret: id=mysecret,src=secret-file
+    username:
+      from_secret: docker_username
+    password:
+      from_secret: docker_password
+```
+
+Using a dockerfile that references the secret-file 
+
+```bash
+# syntax=docker/dockerfile:1.2
+
+FROM alpine
+
+# shows secret from default secret location:
+RUN --mount=type=secret,id=mysecret cat /run/secrets/mysecret
+```
+
+and a secret file called secret-file
+
+```
+COOL BANANAS
+```
+
+
+### Running from the CLI
+
 ```console
 docker run --rm \
   -e PLUGIN_TAG=latest \
@@ -96,4 +136,4 @@ Run the changelog generator again with the future version according to semver.
 docker run -it --rm -v "$(pwd)":/usr/local/src/your-app githubchangeloggenerator/github-changelog-generator -u drone-plugins -p drone-docker -t <secret token> --future-release v1.0.0
 ```
 
-Create your pull request for the release. Get it merged then tag the release.
+Create your pull request for the release. Get it merged then tag the release.