Add basic instructions in README.md for setting up a local dev environment

[yoadsn]

This includes:

• Changes / Additions to README.md
• Add missing dependencies to requirements.txt
• Adding from older repo the .env.example file
• Adding basic .gitignore
• Adding default "long line" Black formatting configuration
• Fix small "empty DB" edge case bug when server first starts

[yoadsn]

@yairl Just saw those PRs were closed without merging. I thought it included useful baseline for contributors - Did I do something wrong?