README updates for macOS

[jspencer]

I don't have hardware yet but I thought I'd run the install and poke around.

For macOS I had to run python3 setup.py develop instead of python setup.py develop

It's possible that stock Macs ship with python3 as the default but I installed (the OS) via migration.
It might be worth adding a note to the README as the error message is not especially helpful:

running develop
error: can't create or remove files in install directory

The following error occurred while trying to add or remove files in the
installation directory:

    [Errno 13] Permission denied: '/Library/Python/2.7/site-packages/test-easy-install-87242.write-test'

The installation directory you specified (via --install-dir, --prefix, or
the distutils default setting) was:

    /Library/Python/2.7/site-packages/

Perhaps your account does not have write access to this directory?  If the
installation directory is a system-owned directory, you may need to sign in
as the administrator or "root" account.  If you do not have administrative
access to this machine, you may wish to choose a different installation
directory, preferably one that is listed in your PYTHONPATH environment
variable.

For information on other options, you may wish to consult the
documentation at:

  https://setuptools.readthedocs.io/en/latest/easy_install.html

Please make the appropriate changes for your system and try again.

Installing on Linux Mint was fine after installing icestorm which was not initially obvious.

[attie]

Python 2.7 isn't supported, but I agree, it might be sensible to mention python3 instead of python - are you willing to make a PR for this?

The error message is indeed not very helpful - this would likely be resolved by using a virtual environment (venv / virtualenv), which I'd suggest is the preferred way to work with Glasgow, and I'd suggest that should be included in the docs too.

Re installing icestorm, did you see and follow this bit in the setup guide?

You will also need Yosys and nextpnr-ice40, both from the master branch. Follow the setup instructions for Yosys and nextpnr.

[jspencer]

OK great, thanks. I'll make a PR. My concern was that there may be cases where python points to Python 3.x and a python3 alias is missing, but from reading around, most people wouldn't have their path and aliases set up that way, so it seems reasonable.

And yes, now I see the note to install IceStorm in the nextpnr README. I must have glossed over that part. Oops!

As for the virtual environment, I'm not familiar enough to write a guide but I'd be more than happy to run it though its paces on a few systems if someone adds that to the readme.