[gthumb] meson: updated installation istructions in README

[Paolo Bacchilega <paobac src gnome org>]

commit c4ac438f3f24c4aef45bc28cada07121f6fdd900
Author: Paolo Bacchilega <paobac src gnome org>
Date:   Sat Oct 21 15:27:49 2017 +0200

    meson: updated installation istructions in README

 README |   12 ++++++------
 1 files changed, 6 insertions(+), 6 deletions(-)
---
diff --git a/README b/README
index 18e1ece..5d9956a 100644
--- a/README
+++ b/README
@@ -119,15 +119,15 @@ Download
 
     http://ftp.gnome.org/pub/GNOME/sources/gthumb
 
-  * If you know what git is and how to use it, you can download and
-    compile the latest development version using the following
-    commands:
+  * You can download and compile the latest development version using the 
+    following commands:
 
         git clone git://git.gnome.org/gthumb
         cd gthumb
-        ./autogen.sh --prefix=/usr CFLAGS="-ggdb"
-        make
-        sudo make install
+        mkdir build && cd build
+        meson ..
+        ninja
+        sudo ninja install
 
     More development information is available at
     https://wiki.gnome.org/Apps/Gthumb/development