Porting libraries (#318)

* initial commit

* Add Immo's discontinued tech section from the blog with some changes

* Add project model overview details

* Add project sytem guidance

* Fix wording about testing in guidance

* Address feedback for libraries doc

* Add multitargeting

* address feedback

* move netframework461 retargeting

* add reverse package search

* feedback pt1

* feedback

* add .net core using xunit

* address feedback

* add disclaimer about csproj

* changes up until project.json

* Updated porting libraries to be .NET Standard-based.

* fix API link

* add metadata

* address feedback

* address feedback

* address feedback

Author: cartermp