Updated Readme.md to include Ubuntu specific requirement

Hi, I was trying to install the package using npm, then tried compiling the source but kept banging my head against the wall. Reviewing the errors I found out I didn't have a necessary lib installed on my (near) default Ubuntu 13.04. So, I believe adding these few lines will help newbs like me out in the future.

I was originally going to write "Linux requires ..." but some distros have it installed by default. Hence, the specific Ubuntu mention. Also, it installs without extra dependencies on my Mac just fine. (Mountain Lion and OpenSSL 0.9.8x)