hub: Add fork and pull-request examples

- Examples follow clone examples as a logical sequence
- Pull-request example stresses that one must first push to own fork

pages/common/hub.md

@@ -11,6 +11,14 @@
 
 `hub clone {{github_username}}/{{repo_name}}`
 
+- Fork repository cloned from another user (creates a remote named after your GitHub username):
+
+`hub fork`
+
+- Create a pull request from your fork:
+
+`git push {{github_username}} && hub pull-request`
+
 - Open the current project's issues page:
 
 `hub browse -- issues`