config: pick a real versioning scheme

I'm pretty sure this is close to what we've been doing, but this
codifies it.

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

config/doc.go

@@ -2,6 +2,18 @@
 // Config type for the main entry point.
 //
 // It's currently meant for reading configs and tested against YAML and JSON.
+//
+// # Version Scheme
+//
+// This package uses an idiosyncratic versioning scheme:
+//
+//   - The major version tracks the input format.
+//   - The minor version tracks the Go source API.
+//   - The patch version increases with fixes and additions.
+//
+// This means that any valid configuration accepted by `v1.0.0` should continue
+// to be accepted for all revisions of the v1 module, but `v1.1.0` may force
+// changes on a program importing the module.
 package config
 
 // This pakcage can't use "omitempty" tags on slices because "not present" and