Update Configuration-File-and-Source-Code-Transformations.markdown

Add XDT feature description
Update Configuration-File-and-Source-Code-Transformations.markdown

fix a typo
Update Configuration-File-and-Source-Code-Transformations.markdown

addressing comments by Luan
Merge pull request #104 from NuGet/nuget-2.5-danliu

Merge XDT transform from Nuget 2.5 danliu