Document PlaintextRenderer and viewsDirectory

[erf]

I recently tried to render a .html file in a route in Vapor 4 and did not find any documentation on it except from on an stackoverflow answer. The community recommended to create an issue.

I got it working by using the PlainTextRenderer

    app.get { req in
        req.view.render("index.html")
    }

and by adding the resourcesDirectory/viewsDirectory

{ProjectPath}/Resources/Views/index.html

I suggest documenting both the PlainTextRenderer and the resourcesDirectory / viewsDirectory.