[gnome-menus] README: Explicitly mention that INSTALL is created by autogen.sh

[Andre Klapper <aklapper src gnome org>]

commit bb7663ae984a449c4b435023f1f324866585215a
Author: Andre Klapper <a9016009 gmx de>
Date:   Sat May 30 13:42:49 2020 +0200

    README: Explicitly mention that INSTALL is created by autogen.sh
    
    Fixes #11

 README | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/README b/README
index 4e85f22..529499f 100644
--- a/README
+++ b/README
@@ -26,7 +26,7 @@ Installation
 
 See the file 'INSTALL'. If you are not using a released version of
 gnome-menus (for example, if you checked out the code from git), you
-first need to run './autogen.sh'.
+first need to run './autogen.sh' to create the 'INSTALL' file.
 
 
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