misc/vim: describe how to get gofmt-on-save behaviour.

Fixes #6826.

R=golang-dev, bradfitz, adg
CC=golang-dev
https://golang.org/cl/31770043

misc/vim/readme.txt

@@ -14,6 +14,11 @@ To use all the Vim plugins, add these lines to your $HOME/.vimrc.
 If you want to select fewer plugins, use the instructions in the rest of
 this file.
 
+A popular configuration is to gofmt Go source files when they are saved.
+To do that, add this line to the end of your $HOME/.vimrc.
+
+  autocmd FileType go autocmd BufWritePre <buffer> Fmt
+
 
 Vim syntax highlighting
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