[docs] Update README to fit template and edit for clarity

Updates README structure to include template sections and TOC. Edits for clarity, flow, and additional information about exec params (for ease of use and to ensure information is available when access to the internet is not).
The flow is a little awkward here. In the template section we say that templates have the added benefit of not requiring escaping '$', and then the next sentence is that variables must be escaped. It does say "in puppet manifests" but I'm not sure the distinction between manifests vs templates is clear enough. You might want to move this note above the "Template" section, and into the "Usage" section.