go-tour: add import unused messages to TODO

TBR=campoy
R=campoy
CC=golang-codereviews
https://golang.org/cl/127720043
diff --git a/TODO b/TODO
index ec4fa3d..a11a4a8 100644
--- a/TODO
+++ b/TODO
@@ -5,6 +5,7 @@
 * Go local
 * Packages
 * Imports
+- "Imported but not used" errors (later perhaps)
 * Exported names
 * Functions
 * Functions continued