Doc generator

Created simple yet terribly convenient script to create the [Helper Overview](https://github.com/chocolatey/chocolatey/wiki/HelpersReference#overview) markdown documentation (and added* existing lines from Wiki to script file comments to keep everything in one place).

*) Also included fix to _Get-BinRoot_ since I need to edit the script anyway

This takes care of the offtopic yet relevant comment by @Nilzor in https://github.com/chocolatey/chocolatey/pull/421#issuecomment-40307678 and every future comment by anyone talking about Helper documentation.