Fill "What is PAQMAN?" and check "Development" Sections in README.md
- [x] Filled "What is PAQMAN?"
- [x] Checked and redone "Development"
- [x] Docker build instructions _(see comments)_
- [x] Staging envs and database presets _(see comments)_
- [x] Mention the proxy directive for the frontend in `package.json`
issue