Fix Issues with the README's Onboarding Flow

[adambechtold]

I walked through the README's onboarding steps and found ways to improve.

• 🐛 Bug - Environment variables did not load for npm run dev

  • 🛠️ Fix - Added an explicit dotenv

• 🤔 Confusion - The rationale behind the SSL_CERT environment variables wasn't clear

  • 📝 Documentation - Comment out SSL_CERT and explain that they are likely not needed
  • 📝 Documentation - Add troubleshooting tips for how to get around the localhost ssl issue with port forwarding

• 📝 Documentation - Link to the pages to set up last.fm and spotify apps

• 📸 Documentation - Replace wireframe with an actual screenshot

[adambechtold]

• 🤔 Confusion - How to update the SESSION_SECRET was unclear.
  • 📝 Documentation - Added comments in the .env.example
  • 📝 Documentation - Added a reminder to change the SESSION_SECRET in the README