docs: add README with usage, config, and API reference

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+# doh-forwarder
+
+A lightweight DNS-over-HTTPS (DoH) forwarder written in Rust.
+
+Accepts DNS queries over HTTPS ([RFC 8484](https://www.rfc-editor.org/rfc/rfc8484)) and forwards them to upstream DoH resolvers (Cloudflare, Google, Quad9, or custom).
+
+## Features
+
+- **RFC 8484 compliant** — GET (`?dns=base64url`) and POST (`application/dns-message`)
+- **Multiple upstreams** — configure several DoH resolvers
+- **Selection strategies** — round-robin or failover
+- **In-memory cache** — TTL-aware with LRU eviction (configurable)
+- **Environment overrides** — override config via env vars
+- **Graceful shutdown** — handles SIGINT and SIGTERM
+- **Health endpoint** — `GET /health`
+
+## Quick Start
+
+```bash
+# Build
+cargo build --release
+
+# Run with default config
+./target/release/doh-forwarder
+
+# Run with a custom config
+./target/release/doh-forwarder --config /path/to/config.toml
+```
+
+## Configuration
+
+Create a TOML config file (see [config/default.toml](config/default.toml)):
+
+```toml
+[server]
+listen = "0.0.0.0:3000"
+
+[upstream]
+resolvers = [
+    { name = "cloudflare", url = "https://cloudflare-dns.com/dns-query" },
+    { name = "google", url = "https://dns.google/dns-query" },
+    { name = "quad9", url = "https://dns.quad9.net/dns-query" },
+]
+strategy = "round-robin"  # or "failover"
+
+[cache]
+enabled = true
+max_entries = 10000
+max_ttl_secs = 3600
+```
+
+### Environment Variable Overrides
+
+| Variable | Description | Example |
+|---|---|---|
+| `DOH_LISTEN` | Override server listen address | `0.0.0.0:8080` |
+| `DOH_UPSTREAM_STRATEGY` | Override upstream strategy | `failover` |
+| `DOH_UPSTREAM_RESOLVERS` | Comma-separated `name=url` pairs | `google=https://dns.google/dns-query,cf=https://cloudflare-dns.com/dns-query` |
+| `RUST_LOG` | Log level filter | `debug`, `doh_forwarder=trace` |
+
+## API
+
+### POST `/dns-query`
+
+Send a raw DNS wire-format query in the request body.
+
+```bash
+# Using a pre-built DNS query (hex-encoded)
+curl -X POST http://localhost:3000/dns-query \
+  -H "Content-Type: application/dns-message" \
+  -H "Accept: application/dns-message" \
+  --data-binary @query.bin
+```
+
+### GET `/dns-query`
+
+Send a base64url-encoded DNS query as a query parameter.
+
+```bash
+# Encode a DNS query and send via GET
+QUERY=$(python3 -c "
+import base64, struct
+# Minimal A record query for example.com
+q = b'\\x12\\x34'  # ID
+q += b'\\x01\\x00'  # flags
+q += b'\\x00\\x01'  # 1 question
+q += b'\\x00\\x00'  # 0 answers
+q += b'\\x00\\x00'  # 0 authority
+q += b'\\x00\\x00'  # 0 additional
+q += b'\\x07example\\x03com\\x00'  # name
+q += b'\\x00\\x01'  # type A
+q += b'\\x00\\x01'  # class IN
+print(base64.urlsafe_b64encode(q).rstrip(b'=').decode())
+")
+curl "http://localhost:3000/dns-query?dns=$QUERY"
+```
+
+### GET `/health`
+
+Returns `ok` if the server is running.
+
+```bash
+curl http://localhost:3000/health
+# ok
+```
+
+## Upstream Strategies
+
+| Strategy | Behavior |
+|---|---|
+| `round-robin` | Distributes queries across resolvers in order |
+| `failover` | Tries resolvers in order; moves to the next on failure |
+
+## Development
+
+```bash
+# Prerequisites
+rustup show            # Rust 1.80+
+
+# Run with auto-reload
+cargo install cargo-watch
+cargo watch -x run
+
+# Run tests
+cargo test
+
+# Lint & format
+cargo clippy -- -D warnings
+cargo fmt --check
+```
+
+See [DEVELOP.md](DEVELOP.md) for full development rules and milestone tracking.
+
+## Project Structure
+
+```
+doh-forwarder/
+├── Cargo.toml
+├── config/
+│   └── default.toml         # Default configuration
+├── src/
+│   ├── main.rs               # Entrypoint, server setup
+│   ├── lib.rs                # Public API re-exports
+│   ├── config.rs             # TOML config + env overrides
+│   ├── cache.rs              # DNS response cache (TTL + LRU)
+│   ├── dns/
+│   │   ├── mod.rs
+│   │   ├── query.rs          # DNS query parsing
+│   │   └── response.rs       # DNS response building
+│   ├── upstream/
+│   │   ├── mod.rs
+│   │   └── doh_client.rs     # DoH forwarding client
+│   └── routes.rs             # HTTP route handlers
+└── tests/
+    └── integration.rs        # Integration tests
+```
+
+## License
+
+MIT