README: fix go get instructions

Currently the instructions tell the user to:

go get -u golang.org/x/image

However, this is not valid because golang.org/x/image contains no Go
files.

Instead it should read:

go get -u golang.org/x/image/...

Fixes golang/go#25618.

Change-Id: I465fac26afdca52878518e4c01d46cf9d12930b9
Reviewed-on: https://go-review.googlesource.com/115675
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 0a312b4..9e12d9e 100644
--- a/README.md
+++ b/README.md
@@ -4,7 +4,7 @@ This repository holds supplementary Go image libraries.
 
 ## Download/Install
 
-The easiest way to install is to run `go get -u golang.org/x/image`. You can
+The easiest way to install is to run `go get -u golang.org/x/image/...`. You can
 also manually git clone the repository to `$GOPATH/src/golang.org/x/image`.
 
 ## Report Issues / Send Patches