Document how to add compiler flags to GradleRIO C++

[spacey-sooty]

In a C++ GradleRIO project we can do something like this in our build.gradle

binaries.all {
  cppCompiler.args "-Werror"
}

This is not documented well at all.
It should probably be documented under the Advanced GradleRIO section.

[ThadHouse]

We can document it, but thats not the correct way we'd want to document. That would break simulation.

nativeUtils.platformConfigs.named('windowsx86-64').configure {
    it.cppCompiler.args.add("/utf-8")
}

Instead the right way is to add flags per platform. And to document what the platforms are.