Add README with circuit diagrams, PCB renders, and project overview - #33

Summary

Adds a detailed README.md for the Battery-Powered ESP32 Air Quality Monitoring
System, along with supporting circuit images.

Changes

• Added README.md with:
  • Project overview and key features
  • Component breakdown (soft power latch, boost converter, ESP32, ADS1115 ADC)
  • System flow diagram (power-on → latch hold → regulation → sensing)
  • Expected simulation results table
  • Reference paper and credits
• Added images/ folder containing:
  • pcb_3d_view.png – 3D PCB render
  • schematic.png – full circuit schematic
  • pcb_layout_front.png – front copper layer routing
  • pcb_layout_back.png – back copper layer routing

Notes

• Images are referenced via relative paths (images/...) so they render
  correctly on GitHub.
• No functional/circuit changes in this PR — documentation only.