add comments

Author: mr-c

setup.py

@@ -50,13 +50,14 @@
       include_package_data=True,
       install_requires=[
           'setuptools',
-          'requests >= 2.6.1',
+          'requests >= 2.6.1',  # >= 2.6.1 to workaround
+          # https://github.com/ionrock/cachecontrol/issues/137
           'ruamel.yaml >= 0.12.4, <= 0.15.51',
           'rdflib >= 4.2.2, < 4.3.0',
           'shellescape >= 3.4.1, < 3.5',
           'schema-salad >= 2.7.20180719125426, < 3',
           'mypy-extensions',
-          'six >= 1.9.0',
+          'six >= 1.9.0',  # >= 1.9.0 required by prov
           'psutil',
           'prov == 1.5.1',
           'bagit >= 1.6.4',