Update documentation for Visual Studio Extension 0.3.0

[pascalberger]

Documentation needs to be updated for Cake Extension for Visual Studio 0.3.0:

Breaking changes

• #115 Remove usage of Experimental option passed to Cake
• #114 Remove support for Visual Studio 2015

Features

• #110 Add support for Visual Studio 2019
• #105 Initial support for overriding the location of cake.exe the TRX uses
• #104 Add solution item template
• #103 Update to new VS2019 SDKs and APIs. Migrate to an AsyncPackage load.
• #79 Add an option to set the "cake.exe" absolute or relative path

Bugs

• #121 Exception when cake.config file doesn't exist
• #97 Unable to add a binding to the Project Open event

Improvements

• #120 Switch to using consistent -- arguments
• #119 Add download commands for additional bootstrapper files
• #118 Add missing preprocessor key words
• #116 Support usage of .Net Tool for Cake
• #101 Update extension to use new VSSDK APIs