vim/README fixes.

+ Expand tabs to spaces.
+ "set modeline" is optional.

vim/README:
    As above.

vim/README

@@ -13,7 +13,6 @@ Mercury Syntax Highlighting Under Vim 6.0+
     filetype on
     filetype plugin on
     syntax enable
-	set modeline
 
 4. It is advisable to have the following file encoding detection lines
 in your ~/.vimrc:
@@ -26,13 +25,19 @@ in your ~/.vimrc:
       endif
     endif
 
-5. If you wish to change the colour scheme used by Vim's syntax highlighting,
+5. Optionally enable modeline detection.  This configures vim to use the
+"vim:" lines at the beginning of some source files to configure settings
+local to that file.
+
+    set modeline
+
+6. If you wish to change the colour scheme used by Vim's syntax highlighting,
 we suggest you put the necessary commands in `~/.vim/colors/mycolours.vim'
 and append the following to the above list of commands in your `~/.vimrc':
 
     colors mycolours
 
-6. Vim will load `~/.vim/syntax/mercury.vim' and `~/.vim/ftplugin/mercury.vim'
+7. Vim will load `~/.vim/syntax/mercury.vim' and `~/.vim/ftplugin/mercury.vim'
 automatically when opening a .m or .moo file.
 
 The syntax file describes two options you may wish to alter from your