[libxml2] Optimize build instructions in README

[Nick Wellnhofer <nwellnhof src gnome org>]

commit fa5e8ca66f226107a87d6c39b3e7820ba3dcba6b
Author: zhouzhongyuan <zhouzhongyuan huawei com>
Date:   Tue Aug 27 19:09:20 2019 +0800

    Optimize build instructions in README
    
    Fixes bug #792181
    https://bugzilla.gnome.org/show_bug.cgi?id=792181
    
    Merge request !40

 README | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/README b/README
old mode 100644
new mode 100755
index 749e671f..5c7bddf8
--- a/README
+++ b/README
@@ -8,6 +8,7 @@ This code is released under the MIT Licence see the Copyright file.
 
 To build on an Unixised setup:
    ./configure ; make ; make install
+   if the ./configure file does not exist, run ./autogen.sh instead.
 To build on Windows:
    see instructions on win32/Readme.txt