doc: add top-level README.md (#16)

Add initial README.md describing the `rbmk` tool.

README.md

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+# Really Basic Measurement Kit
+
+[![GoDoc](https://pkg.go.dev/badge/github.com/rbmk-project/rbmk)](https://pkg.go.dev/github.com/rbmk-project/rbmk)
+
+RBMK (Really Basic Measurement Kit) is a command-line utility
+to facilitate network exploration and measurements. It provides
+two atomic operations (`dig` and `curl`) that you can
+compose together to perform modular network measurements where
+you can observe each operation in isolation.
+
+## Features
+
+- Modular design with separate DNS and HTTP(S) measurements
+- CLI-first approach with composable commands
+- Extensive structured logging for detailed analysis
+- Support for multiple DNS protocols (UDP, TCP, DoT, DoH)
+- HTTP(S) measurements with granular control
+
+The tool is designed to support both general use and measurement-specific
+features, with careful consideration of concurrent operations and
+extensive testing capabilities.
+
+## Installation
+
+```sh
+go install github.com/rbmk-project/rbmk/cmd/rbmk@latest
+```
+
+## Quick Start
+
+```sh
+# Resolve a domain name
+$ rbmk dig +short=ip example.com
+93.184.215.14
+
+# Make an HTTP request
+$ rbmk curl https://example.com/
+
+# Combine dig and curl for step-by-step measurement
+$ IP=$(rbmk dig +short=ip example.com|head -n1)
+$ rbmk curl --resolve example.com:443:$IP https://example.com/
+
+# Collect measurement data in flat JSONL format
+$ rbmk dig --logs dns.jsonl example.com
+$ rbmk curl --logs http.jsonl https://example.com/
+```
+
+For a quick introduction with more examples:
+
+```sh
+$ rbmk intro
+```
+
+## Commands
+
+- `dig`: Performs DNS measurements with a `dig(1)`-like interface
+- `curl`: Measures HTTP/HTTPS endpoints using a `curl(1)`-like interface
+- `intro`: Shows a brief introduction with usage examples
+- `tutorial`: Provides comprehensive usage documentation
+
+Each command supports the `--help` flag for detailed usage information.
+
+## Design
+
+The project focuses on modular, composable measurements where each
+operation that may fail is executed independently. This allows for precise
+analysis of network behavior and easier debugging of issues.
+
+See [DESIGN.md](docs/DESIGN.md) for detailed design documentation.
+
+## Contributing
+
+Contributions are welcome! Please submit pull requests using
+GitHub. Use [rbmk-project/issues](https://github.com/rbmk-project/issues)
+to create issues and discuss features.
+
+## License
+
+```
+SPDX-License-Identifier: GPL-3.0-or-later
+```