[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).
So I had the sentence that way because Puppet can execute powershell commands without this module, e.g:

<pre>
exec { 'c:\windows\system32\windowspowershell\v1.0\powershell.exe ...': }
</pre>

What's unique is that we are adding a new exec provider capable of executing powershell commands without users having to the know the details about how powershell is executed.