Add a caution note to the Getting Started > Installation (Monogame) page warning users about the MonoGame extension templates

[RealCrayfish]

A caution note should be added warning users that if they are using the MonoGame Visual Studio extension they should update their project templates. Many people will have installed the extension and not checked to update the templates as Visual Studio gives no warnings about out-of-date templates and this may cause issues wanting to use the latest release of MonoGame Extended.

While this issue isn't the responsibility of MonoGame Extended, it does causes issues with the library and I think this should be reflected in the documentation.