Added README for the /docs folder

doc/README.md

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+# Ledger Bitcoin App Developer Documentation
+
+This documentation provides detailed information for developers working with the Ledger Bitcoin app.
+
+For many use cases, the code examples provided in the following client libraries might be sufficient to get started:
+- [Python client library](../bitcoin_client)
+- [JavaScript client library](../bitcoin_client_js)
+- [Rust client library](../bitcoin_client_rs)
+
+If you need to go deeper into the rabbit hole 🐇🕳️, refer to the following documents:
+- [bitcoin.md](bitcoin.md): Low-level documentation of the Bitcoin app's communication protocol and commands.
+- [merkle.md](merkle.md): Advanced details on techniques used in the Bitcoin app's secured and scalable communication protocol.
+- [wallet.md](wallet.md): Information on the types of scripts supported by the Ledger Bitcoin app and the security requirements for multi-user or multi-key spending policies.
+- [debugging.md](debugging.md): Guidance on how to diagnose and resolve issues.