Ensure the LICENSE file is part of our wheel (#557)

Also add some comments the other options.
diff --git a/setup.cfg b/setup.cfg
index 14ed149..c22cb95 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -4,12 +4,20 @@
 testpaths = tests
 
 [sdist]
-force_manifest=1
+# Forcibly regenerate the manifest on Python 2.6. This is default behavior on
+# 2.7+ making this option deprecated. This should be removed once we drop 2.6.
+force_manifest = 1
 
 [bdist_wheel]
+# We are a pure-Python project so a single wheel is enough.
 universal = 1
 
+[metadata]
+# Ensure LICENSE is included in wheels.
+license_file = LICENSE
+
 # bdist_rpm settings contributed by Mihai Ibanescu <misa@redhat.com>
+# This is currently *not* actively tested.
 [bdist_rpm]
 release = 1
 build-requires = openssl-devel python-devel python-sphinx