Flesh out the Readme, add comments to example-vars.yml, fix 2 minor types

I fleshed-out the README file to try to make prerequisite steps clear and to provide more detail about what the playbooks do.  I wanted to make things clear for people with little or no Ansible experience (like me, for instance).   **Please make sure that I haven't introduced some problem** in either the information required or (more likely) in the order of the steps needed.  I don't think that that I changed anything along those lines (I didn't add in any commands or introduce anything new), but since I'm truly a naive Ansible user, it's possible.
(I haven't had a chance to run or use Ansible yet.  It's somewhere on my to-do list...)

I also added more comments to the example-var.yml file and re-arranged some of the variables so they were grouped logically.  (I only changed stuff in the "Important Variables" section.)   I standardized the format a bit, thinking that perhaps in the future this file could easily be parsed to generate documentation to put in the readme or in the wiki, etc.

There were 2 really minor spelling typos in comments in the roles/deploy/tasks/main.yml file that I fixed.

Thoughts?  Feedback?