Added comments explaining why coverage is pinned at < 5.0

setup.py

@@ -71,7 +71,16 @@ def package_vars(version_file):
     include_package_data=True,
     install_requires=["docopt", "setuptools"],
     extras_require={
-        "test": ["pre-commit", "pytest", "pytest-cov", "coveralls", "coverage < 5.0"]
+        "test": [
+            "pre-commit",
+            "coveralls",
+            # coveralls does not currently support coverage 5.0
+            # https://github.com/coveralls-clients/coveralls-python/issues/203
+            # is the issue for this on the coveralls project
+            "coverage < 5.0",
+            "pytest-cov",
+            "pytest",
+        ]
     },
     # Conveniently allows one to run the CLI tool as `example`
     entry_points={"console_scripts": ["example = example.example:main"]},