Simplify Vim setup instructions.

globpath assumes comma-separated entries, which $GOPATH does not provide
(it uses : or ; depending on the platform), and this is needlessly
general for most people anyway.

Fixes #14.
diff --git a/README b/README
index 94e970b..9d4e572 100644
--- a/README
+++ b/README
@@ -38,7 +38,8 @@
 Vim
 ---
 Add this to your ~/.vimrc:
-  exe "set rtp+=" . globpath($GOPATH, "src/github.com/golang/lint/misc/vim")
+  set rtp+=$GOPATH/src/github.com/golang/lint/misc/vim
+If you have multiple entries in your GOPATH, replace $GOPATH with the right value.
 
 Running :Lint will run golint on the current file and populate the quickfix list.