bpo-30078: Add an example of passing a path to unittest (#1178)

diff --git a/Lib/unittest/main.py b/Lib/unittest/main.py
index 09fefe1..807604f 100644
--- a/Lib/unittest/main.py
+++ b/Lib/unittest/main.py
@@ -14,6 +14,7 @@
   %(prog)s test_module               - run tests from test_module
   %(prog)s module.TestClass          - run tests from module.TestClass
   %(prog)s module.Class.test_method  - run specified test method
+  %(prog)s path/to/test_file.py      - run tests from test_file.py
 """
 
 MODULE_EXAMPLES = """\