[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).
There is a much better way to handle variable interpolation and powershell commands. The idea is to put all powershell code for `command`, `onlyif`, `unless` parameters into separate templates. Then simply invoke the template function in the resource.

For example, here is a manifest: https://github.com/joshcooper/example-guest/blob/master/manifests/init.pp#L39-L44

<pre>
exec { 'rename-guest':
  command   => template('guest/rename-guest.ps1'),
  onlyif    => template('guest/guest-exists.ps1'),
  provider  => powershell,
  logoutput => true,
}
</pre>

And each template is a powershell script, e.g. https://github.com/joshcooper/example-guest/blob/master/templates/rename-guest.ps1

<pre>
$obj = $(Get-WMIObject Win32_UserAccount -Filter "Name='Guest'")
$obj.Rename("OtherGuest")
</pre>

Note how we don't have to escape `$` in the powershell code.