Improve installation instructions - fix errors about python-in-edu

[aeltanawy]

@shaunagm, for some reason I didn't run into this issue on my Mac however I have developed with postgres before so my computer should have the required setup to run this app locally. The only thing I needed to do though is to build the database with python manage.py migrate so @martin-martin and I thought it would be a good addition to the ReadMe file.

[meg-ray]

Thank you, this is super helpful. I'm updating the issue to specify instructions for multiple platforms. @aeltanawy and @Ben-Hampson

[shaunagm]

Thanks for catching this!

I think these are requirements related to the Spirit forums, which requires postgres and pillow. (This site should use postgres on Heroku and sqlite3 locally.) We should link to their setup instructions regardless so people don't get confused about why the forum's not working if they don't follow the extra steps, but maybe doing the Spirit setup process will avoid some of the trouble you ran into? Or maybe not - I developed on Ubuntu and it may be that the process is just harder on Mac.

I see that @aeltanawy already has a PR up to fix this. Up to you all if/how you want to incorporate this info.

[Ben-Hampson]

I had a similar issue with pip installing psycopg2 on my Mac as well (macOS Mojave 10.14.6).

The error ends with:

    ld: library not found for -lssl
    clang: error: linker command failed with exit code 1 (use -v to see invocation)
    error: command '/usr/bin/gcc' failed with exit code 1
    ----------------------------------------
ERROR: Command errored out with exit status 1:

Full error text: https://gist.github.com/Ben-Hampson/70371c7390e67c6484b91a90778fb266

In previous Django projects to get around this I've pip installed pyscopg2-binary instead and that's worked fine. However that didn't work here. Installing Postgres locally, from postgress.app or homebrew, didn't work either. This command worked for me instead. I'm not sure what it's really about but I wanted to put it here in case it helps others.
env LDFLAGS="-I/usr/local/opt/openssl/include -L/usr/local/opt/openssl/lib" pip install psycopg2

I found this solution at https://stackoverflow.com/questions/26288042/error-installing-psycopg2-library-not-found-for-lssl