Add pkg-config to list of required packets for ubuntu.

Getting-Started.md

@@ -17,10 +17,11 @@ The following packages will need to be installed before you can compile and run
 * libreadline-dev
 * libffi-dev
 * git
+* pkg-config (required at least in ubuntu 14.04)
 
 To install these packages, use the following command:
 
-> sudo apt-get install build-essential libreadline-dev libffi-dev git
+> sudo apt-get install build-essential libreadline-dev libffi-dev git pkg-config
 
 Then, clone the repository to your local machine: