crypto: add import comments.

Change-Id: I33240faf1b8620d0cd600de661928d8e422ebdbc
Reviewed-on: https://go-review.googlesource.com/1235
Reviewed-by: Andrew Gerrand <adg@golang.org>
diff --git a/poly1305/poly1305.go b/poly1305/poly1305.go
index 2270d2b..4a5f826 100644
--- a/poly1305/poly1305.go
+++ b/poly1305/poly1305.go
@@ -16,7 +16,7 @@
 However, in this package AES isn't used and the one-time key is specified
 directly.
 */
-package poly1305
+package poly1305 // import "golang.org/x/crypto/poly1305"
 
 import "crypto/subtle"