ActiveDirectoryDsc: Add CommonModule Markdown Help

[X-Guardian]

Pull Request (PR) description

This PR adds Markdown Help files for the ActiveDirectoryDsc.Common Module.

The Build-ReadMe.ps1 script is used to build the markdown documentation for the common module. this uses functions from the PlatyPS PowerShell Module to build the markdown from the PowerShell comment based help in the module.

This Pull Request (PR) fixes the following issues

• Fixes #377

Task list

• [x] Added an entry to the change log under the Unreleased section of the file CHANGELOG.md. Entry should say what was changed and how that affects users (if applicable), and reference the issue being resolved (if applicable).
• [ ] Resource documentation added/updated in README.md.
• [ ] Resource parameter descriptions added/updated in README.md, schema.mof and comment-based help.
• [x] Comment-based help added/updated.
• [ ] Localization strings added/updated in all localization files as appropriate.
• [ ] Examples appropriately added/updated.
• [ ] Unit tests added/updated. See DSC Community Testing Guidelines.
• [ ] Integration tests added/updated (where possible). See DSC Community Testing Guidelines.
• [ ] New/changed code adheres to DSC Community Style Guidelines.

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This change is 

[codecov[bot]]

Codecov Report

Merging #603 into master will decrease coverage by 1%. The diff coverage is 21%.

@@          Coverage Diff          @@
##           master   #603   +/-   ##
=====================================
- Coverage      99%    98%   -2%     
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  Files          23     24    +1     
  Lines        3045   3091   +46     
=====================================
- Hits         3031   3030    -1     
- Misses         14     61   +47     

[johlju]

Either extending PlatyPS or roll are own in DocGenerator. I separate section in the Wiki would be great.

[johlju]

But agree that this is good as a first step! :)