Update asciidoc instructions to suggest html4 for newer versions

doc/README

@@ -1,21 +1,23 b''
 Mercurial's documentation is currently kept in ASCIIDOC format, which
 is a simple plain text format that's easy to read and edit. It's also
 convertible to a variety of other formats including standard UNIX man
 page format and HTML.
 
 To do this, you'll need to install ASCIIDOC:
 
  http://www.methods.co.nz/asciidoc/
 
 To generate the man page:
 
  asciidoc -d manpage -b docbook hg.1.txt
  xmlto man hg.1.xml
 
 To display:
 
  groff -mandoc -Tascii hg.1 | more
 
 To create the html page (without stylesheets):
 
- asciidoc -b html hg.1.txt
+ asciidoc -b html4 hg.1.txt
+
+(older asciidoc may want html instead of html4 above)