Update intro_installation.rst

This is different wording for using the PPA to install.  I also added the installation of apt-add-repository package which isn't always installed.

(I'm a newbie but while running these I wished whatever style was being used on the docs pages would have let me  cut and paste the commands without having to edit out the $ characters manually....)
Merge pull request #7698 from gsker/patch-1

Update intro_installation.rst