Update readme, changelog and install instructions

README.md

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+Antigravitaattori
+=================
+
+Multiplayer flying saucer racing game.
+
+This is a forked version of the original game, featuring the following changes:
+
+ - ported to libpng 1.6
+ - ported to SDL 2.0
+ - dropped autotools, simplified build system
+ - support for game controllers (tested with wireless XBOX 360 controller)
+
+## Requirements
+ - SDL2
+ - libpng
+ - libalut
+ - libglu
+ - libopenal
+
+### Ubuntu 16.04
+
+	$ apt-get install make g++ libsdl2-dev libalut-dev libglu-dev libopenal-dev
+
+## Build and install
+
+Build and install with make:
+
+	$ make && make install
+
+By default, the game is installed to `/usr/local`. This can be changed with
+variable `PREFIX`, e.g:
+
+	$ make PREFIX=/usr && make install PREFIX=/usr