Update user guide

Add labal management description in user guide

docs/user_guide.md

@@ -6,6 +6,7 @@ This guide walks you through the fundamentals of using Harbor. You'll learn how
 * [Manage members of a project.](#managing-members-of-a-project)
 * [Replicate projects to a remote registry.](#replicating-images)
 * [Search projects and repositories.](#searching-projects-and-repositories)
+* [Manage labels.](#managing-labels)
 * [Manage Harbor system if you are the system administrator:](#administrator-options)
   * [Manage users.](#managing-user)
   * [Manage endpoints.](#managing-endpoint)
@@ -161,6 +162,31 @@ Entering a keyword in the search field at the top lists all matching projects an
 
 ![browse project](img/new_search.png)
 
+## Managing labels
+Harbor provides two kinds of labels to isolate kinds of resources(only images for now):
+* **Global Level Label**: Managed by system administrators and used to manage the images of the whole system. They can be added to images under any projects.
+* **Project Level Label**: Managed by project administrators under a project and can only be added to the images of the project.
+
+### Managing global level labels
+The system administrators can list, create, update and delete the global level labels under `Administration->Configuration->Labels`:
+
+![manage global level labels](img/manage_global_level_labels.png)
+
+### Managing project level labels
+The project administrators and system administrators can list, create, update and delete the project level labels under `Labels` tab of the project detail page:
+
+![manage project level labels](img/manage_project_level_labels.png)
+
+### Adding labels to/remove labels from images
+Users who have system administrator, project administrator or project developer role can click the `ADD LABELS` button to add labels to or remove labels from images. The label list contains both globel level labels(come first) and project level labels:
+
+![add labels to images](img/add_labels_to_images.png)
+
+### Filtering images by labels
+The images can be filtered by labels:
+
+![filter images by labels](img/filter_images_by_label.png)
+
 ## Administrator options  
 ### Managing user  
 Administrator can add "Administrator" role to one or more ordinary users by checking checkboxes and clicking `SET AS ADMINISTRATOR`. To delete users, checked checkboxes and select `DELETE`. Deleting user is only supported under database authentication mode.