[opus] Add CMake build instructions to README

[Ralph Giles]

On Wed, 2020-06-24 at 17:59 +0000, Marcus Asteborg wrote:

> I am not so familiar with automake and nix systems.
> Does the configure install make and compilers that is needed for the
> make step?
> Or is it assumed that the nix system already has it installed?

It's assumed the compiler is already installed. If not, the configure
script will report and error, as it does for any required library
dependencies, and the developer can install the missing pieces and try
again.

 -r