Update the WPF developer guide

WPF should have an easily identifiable link "How to Build WPF"

This information should be captured:

Install the most recent version of the .NET SDK (master 6.0.x runtime)
- dotnet/installer: .NET SDK Installer (github.com)

Install git for Windows
- Git for Windows
- Accept all the default options

Install VS Preview (WPF requires the most recent VS Preview)
Open an administrator command prompt
Clone the repo
- git clone https://github.com/dotnet/wpf.git
- the WPF source will be cloned to \wpf by default

Start 'Add or Remove Programs' from the start menu
Select Visual Studio Enterprise 2019 Preview
Under Installed see 'Visual Studio Enterprise 2019 Preview'
Select 'More' and 'Import Configuration'
In the WPF source, choose the file \wpf\documentation\wpf.vsconfig
- This contains all the required VS components for building WPF
Click 'Review Details'
Click lower right hand corner to start installation

Build the WPF framework:
build /p:Platform=x64 -pack (for x64 machines)
build /p:Platform=x86 -pack (for x86 machines)