Add instructions for targeting .NET Core with MSBuild projects (dotnet/corefx#9403)

Add instructions for targeting .NET Core with MSBuild projects (dotnet/corefx#9403)

* Add step by step instructions for using MSBuild to target .NET Core
* Rearrange and edit instructions for targeting .NET Core with MSBuild
* Link to updated NuGet extension for VS, add instructions allowing PCL/dnxcore50 packages to be referenced
* Reformatting
* Update existing getting started instructions
* Change prerequisite to VS 2015 Update 3, which fixes error that AnyCPU isn't supported
* Add comments for MSBuild properties
* Recommend project.json for new projects
* Add CoreApp as another recommendation to start with
* Start with Console app template instead of portable library, use Microsoft.NETCore.App package
* Fix indentation

Commit migrated from https://github.com/dotnet/corefx/commit/79a7c578cce7cf82eb4c2bf6d0b0fb5dba2cb895