[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).
@joshcooper I've put your example in as an alternative way, which you can see here: https://github.com/puppetlabs/puppetlabs-powershell/pull/19/files#diff-04c6e90faac2675aa89e2176d2eec7d8R54