[opus] Add CMake build instructions to README

[Mark Harris]

On Sat, Jun 13, 2020 at 2:59 PM Marcus Asteborg <xnorpx at outlook.com> wrote:
>
> Hi,
>
> Here is a patch to add build instructions for CMake to the README.
>
> I tried to follow the previous automake style instructions as closely as possible but added instructions for Windows.
> Noted that it is still experimental
>
> //Marcus


Hi Marcus,

Currently the build instructions start with simple instructions for
building from the release tarball, and then provide more complex
alternative instructions that can be followed for those that need to
build the latest development code from git.  This adds instructions
for building with CMake, but not from the normal release tarball as
expected; instead the CMake instructions are a variation of the
alternative instructions for building from git.

If there is a reason that CMake should only be used with git then that
should be explained in the instructions; otherwise I would expect the
instructions to apply to the normal release tarball.  It's not clear
whether there is also a need for a fourth set of build instructions
for using CMake in combination with git, unless there is something
particularly notable about that combination.  Perhaps that might be
covered just for Windows, as an addendum to the existing git
instructions, since the existing git instructions specifically cover
several platforms, but not Windows.

 - Mark