encoding/ianaindex: add comment about being a work in progress.
encoding/ianaindex: add comment about being a work in progress.

Updates golang/go#18278

Change-Id: I1713bff63db5e68e074043f8f34c4998a4ab1275
Reviewed-on: https://go-review.googlesource.com/34530
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/text