help config: explain that config files do not exist by default

Inspired by critique given on StackOverflow where a user writes:

  I can have a good guess at what "%USERPROFILE%" might signify but
  none of the files listed in the "hg help config" output exist after
  running the installer. Previous experience would suggest that
  missing files mean something somewhere has gone seriously wrong.

  http://stackoverflow.com/questions/2329023/2351139#2351139

mercurial/help/config.txt

@@ -22,6 +22,12 b' On Unix, these files are read:'
 - ``<install-root>/etc/mercurial/hgrc``
 - ``<install-root>/etc/mercurial/hgrc.d/*.rc``
 
+These files do not exist by default and you will have to create the
+appropriate configuration files yourself: global configuration like
+the username setting is typically put into
+``%USERPROFILE%\mercurial.ini`` or ``$HOME/.hgrc`` and local
+configuration is put into the per-repository ``<repo>/.hg/hgrc`` file.
+
 If there is a per-repository configuration file which is not owned by
 the active user, Mercurial will warn you that the file is skipped::