update user guide

docs/user_guide.md

@@ -4,9 +4,14 @@ This guide takes you through the fundamentals of using Harbor. You'll learn how
 
 * Manage your projects.
 * Manage members of a project.
+* Replicate projects to a remote registry.
 * Search projects and repositories.
-* Manage Harbor system if you are the system administrator.
+* Manage Harbor system if you are the system administrator:
+ + Manage users.
+ + Manage targets.
+ + Manage policies.
 * Pull and push images using Docker client.
+* Delete repositories.
 
 
 ##Role Based Access Control
@@ -60,6 +65,9 @@ You can update or remove a member by clicking the icon on the right.
 
 ![browse project](img/remove_update_member.png)
 
+##Replicating images
+If you are a system administrator, you can replicate images to a remote registry. 
+
 ##Searching projects and repositories
 Entering a keyword in the search field at the top lists all matching projects and repos. The search result includes public repos and private repos you have access privilege to.  
 
@@ -71,6 +79,9 @@ Administrator can add "SysAdmin" role to an ordinary user by toggling the switch
 
 ![browse project](img/set_admin_remove_user.png)
 
+###Image replication
+
+
 ##Pulling and pushing images using Docker client
 
 **NOTE: Harbor only supports Registry V2 API. You need to use Docker client 1.6.0 or higher.**  
@@ -120,3 +131,20 @@ $ docker push 10.117.169.182/demo/ubuntu:14.04
 ```  
 
 **Note: Replace "10.117.169.182" with the IP address or domain name of your Harbor node.**
+
+##Deleting repositories
+
+Repositories deletion runs in two steps.  
+First, delete repositories in Harbor's UI. This is soft deletion. You can delete the entire repository or just a tag of it.  
+
+**Note: If both tag A and tag B reference the same image, after deleting tag A, B will also disappear.**  
+
+Second, delete the real data using registry's garbage colliection(GC). 
+
+```sh
+$ docker run -it --name gc -v /harbor_deploy_path/harbor/Deploy/config/registry/:/etc/registry/ -v /data/registry/:/storage/ registry:2.4.0 garbage-collect [--dry-run] /etc/registry/config.yml
+```  
+Replace "/harbor_deploy_path" with path where your Harbor is deployed. And if your images are not storaged in "/data/registry", replace it too.  
+Option "--dry-run" will print the progress without removing any data.
+
+About the details of GC, please see [GC](https://github.com/docker/distribution/blob/master/docs/garbage-collection.md)