go.image/tiff: add missing encoder to pkg doc

R=golang-dev, nigeltao
CC=golang-dev
https://golang.org/cl/13302045

 tiff/reader.go | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tiff/reader.go b/tiff/reader.go
index d64aa59..bd25c3c 100644
--- a/tiff/reader.go
+++ b/tiff/reader.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package tiff implements a TIFF image decoder.
+// Package tiff implements a TIFF image decoder and encoder.
 //
 // The TIFF specification is at http://partners.adobe.com/public/developer/en/tiff/TIFF6.pdf
 package tiff