Add library to installation instructions

Trying to use the qrencoder gem on Ubuntu 12.04 per the current README kept giving me the following error: /home/.../custom_require.rb:36:in `require': libqrencode.so.3: cannot open shared object file: No such file or directory - /home/.../qrencoder-1.4.1/lib/qrencoder/qrencoder_ext.so (LoadError)

A SO post recommended the library for using qrencode with python, and it fixed my issue, would propose updating the *nix install instructions to help other users.