Suggest using yum to install Ansible throughout all of the content.

[tima]

Using pip is fine if you're only going to use Ansible engine on a box, however; this will conflict with any installation of Ansible Tower or AWX on the same server and should be avoided. We need to make that clear by suggesting yum and placing appropriate caveats to why they don't want to use pip.

[tima]

Relates to #85 and #151.