[shotwell] Update INSTALL

[Jens Georg <jensgeorg src gnome org>]

commit a44252bc0339bfc0afc0721ba96c34f77c5aadee
Author: Jens Georg <mail jensge org>
Date:   Fri Jul 29 00:14:16 2016 +0200

    Update INSTALL
    
    Signed-off-by: Jens Georg <mail jensge org>

 INSTALL |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/INSTALL b/INSTALL
index dd54ec7..3fd6efa 100644
--- a/INSTALL
+++ b/INSTALL
@@ -4,14 +4,15 @@
 
     To build and install Shotwell, type the following commands:
 
+    $ [ -f configure ] || ./autogen.sh
     $ ./configure
     $ make
-    $ make install
+    $ Optional: make install
 
     * By default, Shotwell will install under /usr/local.  The configure script
       can customize the prefix directory.  Run ./configure --help for
       instructions and other installation options.
-  
+
     * You may need to execute "make install" as root (i.e. sudo make install)
       if installing to system directories.