macOS: 'brew install openjpeg' for JPEG2000 support

[hugovk]

Shall we include JPEG2000 as well?

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Whilst we're here, a line or two down we tell the user to pip install Pillow from PyPI, or instead build from source:

Now install Pillow with::

    python3 -m pip install --upgrade pip
    python3 -m pip install --upgrade Pillow

or from within the uncompressed source directory::

    python3 -m pip install .

But this is the section on Building from source, we already had the Basic installation near the top of the page.

Shall we change it to this?

Now install from within the uncompressed source directory::::

    python3 -m pip install --upgrade pip
    python3 -m pip install .

[radarhere]

Good point about building from source.

But there are three other instances where we tell people to pip install --upgrade Pillow when "Building from source", and I wonder if it's an extra step to tell them to download the source code in each section. How about using --no-binary :all: instead? See PR #6066