docs: add simple readme

closes #23

Author: alukach

README.md

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+# Multistore
+
+A set of Rust crates for building multi-runtime S3 gateway proxies. Multistore provides the components — S3 request parsing, SigV4 authentication, backend resolution, configuration, and middleware — to assemble a proxy that sits between S3-compatible clients and backend object stores. Combine them into a native Tokio/Hyper server, a Cloudflare Worker at the edge, or your own custom runtime.
+
+> [!WARNING]
+> This project is in early stages and should be considered experimental. APIs and interfaces may change significantly between versions.
+
+## Features
+
+- **Unified interface** — Single stable URL per dataset regardless of backend provider. Backend migrations are invisible to data consumers.
+- **Native S3 compatibility** — Works with aws-cli, boto3, DuckDB, obstore, GDAL, or any S3-compatible client. Just set the endpoint URL.
+- **Flexible authentication** — Anonymous access, long-lived access keys, or OIDC/STS token exchange for temporary credentials.
+- **Multi-runtime** — Same core logic deploys as a native server in containers or as a Cloudflare Worker at the edge.
+- **Zero-copy streaming** — Presigned URLs enable direct streaming between clients and backends without buffering.
+- **Extensible** — Pluggable traits for request resolution, configuration, and backend I/O.
+
+## Crate Layout
+
+```
+crates/
+├── core/              (multistore)                # Runtime-agnostic: traits, S3 parsing, SigV4, registries
+├── metering/          (multistore-metering)       # Usage metering and quota enforcement middleware
+├── sts/               (multistore-sts)            # OIDC/STS token exchange
+├── oidc-provider/     (multistore-oidc-provider)  # Outbound OIDC provider
+├── static-config/     (multistore-static-config)  # Static config provider
+├── path-mapping/      (multistore-path-mapping)   # Path mapping utilities
+└── cf-workers/        (multistore-cf-workers)     # Cloudflare Workers runtime (WASM)
+
+examples/
+├── server/            (multistore-server)         # Tokio/Hyper native server
+├── lambda/            (multistore-lambda)         # AWS Lambda runtime
+└── cf-workers/        (multistore-cf-workers)     # Cloudflare Workers
+```
+
+## Getting Started
+
+See the [full documentation](https://developmentseed.org/multistore/getting-started/) for configuration, deployment, and usage guides.
+
+## Development
+
+```bash
+# Format
+cargo fmt
+
+# Lint
+cargo clippy --fix --allow-dirty --allow-staged
+
+# Check (native)
+cargo check
+
+# Check (WASM)
+cargo check -p multistore-cf-workers --target wasm32-unknown-unknown
+```
+
+## License
+
+[MIT](LICENSE)