Document authentication with porcelain.clone

docs/tutorial/porcelain.txt

@@ -24,6 +24,10 @@ Clone a repository
 ------------------
 
   >>> porcelain.clone("git://github.com/jelmer/dulwich", "dulwich-clone")
+  
+Authentication works using the ``username`` and ``password`` parameters:
+
+  >>> porcelain.clone("https://example.com/a-private-repo.git", "a-private-repo-clone", username="user", password="password")
 
 Commit changes
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