README.md: update screenshot and installation

Instruction how to install form source and add a screenshot.

Author: gicmo

README.md

@@ -2,11 +2,27 @@
 
 [GameMode][gamemode] is a software package to "optimize Linux system
 performance on demand". This GNOME Shell extension will show the current
-status of GameMode.
+status of GameMode. For it to work, GameMode 1.4 or greater is required.
+
+![screenshot][screenshot]
+
+## Installation from source
+
+Obtain the current development version of GameMode extension via:
+
+	git clone https://github.com/gicmo/gamemode-extension.git
+
+A script is included to create a zip archive which also can also install
+the extension for the current user.
+
+	./make-zip.sh         # just create the archive
+	./make-zip.sh install # create the archive and install it
+
 
 ## License
 The GameMode GNOME Shell extension is distributed under the terms of the
 GNU Lesser General Public License, version 2.1 or later. See the
 COPYING file for details.
 
 [gamemode]: https://github.com/FeralInteractive/gamemode
+[screenshot]: https://github.com/gicmo/gamemode-extension/raw/master/screenshot/gamemode.png