Document installation process

[federico3]

• Set up machine (unattended-upgrades, etc.)
• Pull this repo
• requirements.txt
• apt-get stuff
• Set RTC time
• Install all required data files
• Set up service to download Celestrak files by copying fetch_celestrak_data.{timer,service} to /etc/systemd/system, then sudo systemctl daemon-reload, then sudo systemctl enable, then sudo systemctl start.
• Set up service to autostart trovastelle.
• Set up LED pins at startup