Add DTR docs with history

docker-trusted-registry/reference/install.md

@@ -9,6 +9,7 @@ menu:
 title: install
 ---
 
+
 # docker/dtr install
 
 Install Docker Trusted Registry on this Docker Engine

docker-trusted-registry/reference/join.md

@@ -13,6 +13,14 @@ title: join
 
 Add a new replica to an existing DTR cluster
 
+## Usage
+
+```bash
+$ docker run -it --rm docker/dtr \
+   join [command options]
+```
+
+
 ## Description
 
 This command installs DTR on the Docker Engine that runs the command,