add ruby install instructions

Author: Jan Schulte

README.md

@@ -30,5 +30,8 @@ sudo apt-get install build-essential libssl-dev libyaml-dev libreadline-dev open
 Install Ruby
 
 ```bash
-wget http://cache.ruby-lang.org/pub/ruby/2.1/ruby-2.1.5.tar.gz
+$ wget http://cache.ruby-lang.org/pub/ruby/2.1/ruby-2.1.5.tar.gz
+$ ./configure
+$ make
+$ sudo make install
 ```