docs: add readme and contribution docs

CONTRIBUTING.md

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+# Contributing
+
+Thanks for your interest in improving MAL.
+
+## Before you start
+
+- Open an issue first for large changes so scope is clear
+- Keep pull requests focused and small when possible
+- Prioritize user-facing clarity: cleaner flows, less friction, better defaults
+
+## Local setup
+
+```bash
+# install templ CLI
+go install github.com/a-h/templ/cmd/templ@latest
+
+# generate templates
+templ generate
+
+# run tests
+go test ./...
+
+# run app
+go run ./cmd/server
+```
+
+## Development guidelines
+
+- Follow existing folder boundaries (`internal/features/*`, `internal/jikan`, `internal/templates`)
+- Prefer simple, explicit solutions over broad abstractions
+- Do not add dependencies unless there is a clear benefit
+- Keep generated files in sync when changing `.templ` or SQL query definitions
+
+## Pull request checklist
+
+- Explain the user problem this change solves
+- Describe tradeoffs or constraints
+- Include before/after behavior notes
+- Ensure `go test ./...` passes locally
+
+## Security
+
+- Never commit secrets, private tokens, or real credentials
+- Keep `.env` values local
+- Report security issues privately before public disclosure