Adding some helpful comments to the noxfile.

conftest.py

@@ -1,3 +1,17 @@
+# Copyright 2016 Google Inc. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
 import os
 
 import pytest

nox.py

@@ -1,3 +1,17 @@
+# Copyright 2016 Google Inc. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
 import fnmatch
 import os
 import tempfile
@@ -86,8 +100,12 @@ def session_tests(session, interpreter, extra_pytest_args=None):
     session.install(REPO_TOOLS_REQ)
     session.install('-r', 'requirements-{}-dev.txt'.format(interpreter))
 
+    # extra_pytest_args can be send by another session calling this session,
+    # see session_travis.
     pytest_args = COMMON_PYTEST_ARGS + (extra_pytest_args or [])
 
+    # session.posargs is any leftover arguments from the command line, which
+    # allows users to run a particular test instead of all of them.
     for sample in (session.posargs or SAMPLES):
         session.run(
             'py.test', sample,