add .coveragerc example

doc/source/ch06_unittest.rst

@@ -68,6 +68,33 @@ The coverage info is shown, and a coverage report in html format will be generat
 .. _pytest: https://docs.pytest.org/en/7.1.x
 .. _coverage: https://coverage.readthedocs.io/en/6.5.0/
 
+Exclude files
+-------------
+
+To exclude specific files from coverage report, write a `.coveragerc` file.
+
+.. code-block::
+
+    [run]
+    omit =
+        # omit anything in a .local directory anywhere
+        */.local/*
+        # omit everything in /usr
+        /usr/*
+        # omit this single file
+        utils/tirefire.py
+
+An example `.coveragerc` for `csst_proto` is
+
+.. code-block::
+
+    [run]
+    omit =
+        tests/*
+        setup.py
+        csst_proto/top_level_interface.py
+        test_codestyle.py
+
 
 Conventions
 -----------