Update build instructions in the README

[philipp-classen]

The steps to setup Python, download data from S3, and build the website need to be updated. In the meantime, something like that should work:

1. Download data as described here: https://github.com/whotracksme/whotracks.me/blob/master/whotracksme/data/assets/README.md

2. Build the website:

   Create a venv: python3.10 -m venv venv
   . venv/bin/activate
   pip install -r requirements-dev.txt
   pip install -e '.[dev]'
   pytest (should pass now)
   whotracksme website