add dev quickstart to README.

Document env copy, no-cache compose build, and local npm install for editors.

README.md

@@ -1,3 +1,20 @@
 # Private Image
 
 Self-hosted, privacy-focused image hosting. Images are metadata-stripped and encrypted in the browser before upload. Supports **zero-knowledge** viewer links and **hotlink** mode for embeds.
+
+## Development
+
+### Start dev environment
+
+```bash
+cp .env.example .env.dev
+
+docker compose --env-file .env.dev -f docker-compose.dev.yml build --no-cache
+docker compose --env-file .env.dev -f docker-compose.dev.yml up --force-recreate
+```
+
+### Install npm packages for code editor visibility
+
+```bash
+cd backend && npm i && cd ../frontend && npm i
+```