[gexiv2] Update build instructions for building from GIT

[Jens Georg <jensgeorg src gnome org>]

commit 876ac507086d5baa7b4e284cd67193b018d62310
Author: Jens Georg <mail jensge org>
Date:   Sat Apr 30 11:02:07 2016 +0200

    Update build instructions for building from GIT
    
    Signed-off-by: Jens Georg <mail jensge org>
    
    https://bugzilla.gnome.org/show_bug.cgi?id=749800

 INSTALLING |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/INSTALLING b/INSTALLING
index 4ed1fc2..2c3b4a1 100644
--- a/INSTALLING
+++ b/INSTALLING
@@ -8,6 +8,9 @@
     $ make
     $ make install
 
+    When building from git, you need to run ./autgen.sh to generate the
+    configure script.
+
     * By default, gexiv2 will install under /usr/local.  The configure script
       can customize the prefix directory.  Run ./configure --help for
       instructions and other installation options.