Update instructions on "Running the app"

[agarny]

I followed the instructions (in README.md) to run the app, but being on macOS and using Xcode 12, it didn't work. However, what works is the following:

python3 -m venv ./venv
. ./venv/bin/activate
pip install --upgrade pip
pip install wheel                       <-- Added to prevent several warnings when installing the requirements
export ARCHFLAGS="-arch x86_64"         <-- Needed on macOS when using Xcode 12
pip install -r requirements.txt
pip install -r requirements-dev.txt
gunicorn main:app