mention apt repo in install instructions (fix #22)

README.rst

@@ -109,11 +109,14 @@ How to install vcstool?
 On Debian-based platforms the recommended method is to install the package *python-vcstool*.
 On Ubuntu this is done using *apt-get*::
 
+  sudo sh -c 'echo "deb http://packages.ros.org/ros/ubuntu $(lsb_release -sc) main" > /etc/apt/sources.list.d/ros-latest.list'
+  sudo apt-key adv --keyserver hkp://pool.sks-keyservers.net --recv-key 0xB01FA116
+  sudo apt-get update
   sudo apt-get install python-vcstool
 
 On other Systems, use the `PyPI <http://pypi.python.org>`_ package::
 
-  pip install vcstool
+  sudo pip install vcstool
 
 
 Setup auto-completion