[m-rev.] for post-commit review: update and rewrite README.CSharp

[Peter Wang]

On Thu, 19 Jan 2023 20:30:27 +1100 Julien Fischer <jfischer at opturion.com> wrote:
> 
> Update and rewrite README.CSharp.

> +Compiling programs with the `csharp` grade
> +------------------------------------------
> 
> -Note that when building programs using the csharp grade you *must* use
> -mmc --make.
> +Once you have a Mercury installation that includes the `csharp` grade, you
> +can build programs such as `hello.m` of `calculator.m` in the [samples](samples)
> +directory.

I fixed this typo. The rest looks fine.

Peter