fix instructions how to update manual on www.gnu.org

[Bruno Haible]

Trying to regenerate the manual using the instructions from doc/README,
I get an error:

Generating dvi... (env LANG= LC_MESSAGES= LC_ALL= LANGUAGE= texi2dvi  -t 
@finalout "[...]/gnulib-git/doc/gnulib.texi")
[...]/bin/texi2dvi: texinfo.tex appears to be broken.
This may be due to the environment variable TEX set to something
other than (plain) tex, a corrupt texinfo.tex file, or 
to tex itself simply not working.
This is pdfTeX, Version 3.14159265-2.6-1.40.16 (TeX Live 2015/Debian) 
(preloaded format=etex)
 restricted \write18 enabled.
entering extended mode
(./txiversion.tex
! I can't find file `texinfo.tex'.
l.1 \input texinfo.tex 
                       @bye
(Press Enter to retry, or Control-D to exit)
Please type another input file name: 
! Emergency stop.
l.1 \input texinfo.tex 
                       @bye
No pages of output.
Transcript written on txiversion.log.
[...]/bin/texi2dvi: quitting.


This fixes it.


2017-05-10 Bruno Haible  <address@hidden>

        Fix instructions how to update manual on www.gnu.org.
        * doc/README: Add -I option, so that texi2dvi finds texinfo.tex.

diff --git a/doc/README b/doc/README
index 00d1c35..0ffae93 100644
--- a/doc/README
+++ b/doc/README
@@ -26,6 +26,7 @@ How to update gnulib manual on www.gnu.org
    cd $GNULIB_CHECKOUT/doc
    make updated-stamp
    ../build-aux/gendocs.sh \
+     -I ../build-aux \
      --email address@hidden \
      -o "$GNULIB_WWW_CHECKOUT/manual" \
      gnulib "The GNU Portability Library"