misc/vim: clarifies syntax highlighting installation instructions

Based on the review of CL 1723044, I've changed the installation instructions
for the vim syntax files to suggest symlinking the files rather than copying
the files. Also the wording has changed to be more consistent.

R=golang-dev, Kyle Lemons, adg
CC=golang-dev
https://golang.org/cl/1702045

misc/vim/readme.txt

@@ -3,10 +3,21 @@ Vim syntax highlighting for Go (http://golang.org)
 
 To install automatic syntax highlighting for GO programs:
 
-  1. Copy ftplugin/gofiletype.vim to the ftplugin directory underneath your vim
-     runtime directory (normally $HOME/.vim/ftplugin)
-  2. Copy syntax/go.vim to the syntax directory underneath your vim runtime
-     directory (normally $HOME/.vim/syntax)
-  3. Add the following line to your .vimrc file (or equivalent):
-    
+  1. Copy or link the filetype detection script to the ftdetect directory
+     underneath your vim runtime directory (normally $HOME/.vim/ftdetect)
+  2. Copy or link syntax/go.vim to the syntax directory underneath your vim
+     runtime directory (normally $HOME/.vim/syntax). Linking this file rather
+     than just copying it will ensure any changes are automatically reflected
+     in your syntax highlighting.
+  3. Add the following line to your .vimrc file (normally $HOME/.vimrc):
+
      syntax on
+
+In a typical unix environment you might accomplish this using the following
+commands:
+
+  mkdir -p $HOME/.vim/ftdetect
+  mkdir -p $HOME/.vim/syntax
+  ln -s $GOROOT/misc/vim/ftdetect/gofiletype.vim $HOME/.vim/ftdetect/
+  ln -s $GOROOT/misc/vim/syntax/go.vim $HOME/.vim/syntax
+  echo "syntax on" >> $HOME/.vimrc