misc/vim: update installation instructions to work better with some Linux distributions.

Fixes #3308.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/7301086

misc/vim/readme.txt

 Vim plugins for Go (http://golang.org)
 ======================================
 
-To use all the Vim plugins, add these lines to your vimrc.
+To use all the Vim plugins, add these lines to your $HOME/.vimrc.
 
-  set rtp+=$GOROOT/misc/vim
+  " Some Linux distributions set filetype in /etc/vimrc.
+  " Clear filetype flags before changing runtimepath to force Vim to reload them.
+  filetype off
+  filetype plugin indent off
+  set runtimepath+=$GOROOT/misc/vim
   filetype plugin indent on
   syntax on