config: add top-level docs

Signed-off-by: Hank Donnay <hdonnay@redhat.com>

config/doc.go

@@ -0,0 +1,10 @@
+// Package config is the configuration package for Clair's binaries. See the
+// Config type for the main entry point.
+//
+// It's currently meant for reading configs and tested against YAML and JSON.
+package config
+
+// This pakcage can't use "omitempty" tags on slices because "not present" and
+// "empty" aren't distinguished. This would be much easier if code didn't
+// serialize our config struct. It's impossible to implement custom YAML
+// marshalling without importing the yaml.v3 package.