Merge pull request #759 from ChipmunkV/ubuntu-16.04

doc: Ubuntu 16.04 requirement

Author: TheJJ

doc/build_instructions/ubuntu_15.04.md

@@ -0,0 +1,4 @@
+# Prerequisite steps for Ubuntu users (Ubuntu 16.04)
+
+ - `sudo apt-get update`
+ - `sudo apt-get install cmake libfreetype6-dev python3-dev cython3 libepoxy-dev libsdl2-dev libsdl2-image-dev libopusfile-dev libfontconfig1-dev libharfbuzz-dev opus-tools python3-pil python3-numpy python3-pygments qtdeclarative5-dev qml-module-qtquick-controls`

doc/build_instructions/ubuntu_16.04.md

@@ -61,7 +61,7 @@ There are some prerequisite steps that need to be performed so *openage* can be
 built successfully. Those steps vary from platform to platform, and are
 described below for some of the most common ones:
 
-- [Ubuntu (>= 15.04)](build_instructions/ubuntu_15.04.md)
+- [Ubuntu (>= 16.04)](build_instructions/ubuntu_16.04.md)
 - [Fedora 20, 21](build_instructions/fedora_20_21.md)
 - [Fedora 22](build_instructions/fedora_22.md)
 - [openSUSE 13.2](build_instructions/opensuse_13.2.md)