Document selectable crate features

List each chain, storage, payment, and binding feature and explain
the unchanged native defaults. Show both lean and custom binding builds
so optional backend dependencies can be selected deliberately.

Co-Authored-By: HAL 9000

Author: tnull

README.md

@@ -63,10 +63,60 @@ LDK Node currently comes with a decidedly opinionated set of design choices:
 
 - On-chain data is handled by the integrated [BDK][bdk] wallet.
 - Chain data may currently be sourced from the Bitcoin Core RPC interface, or from an [Electrum][electrum] or [Esplora][esplora] server.
-- Wallet and channel state may be persisted to an [SQLite][sqlite] or [PostgreSQL][postgresql] database, to file system, or to a custom back-end to be implemented by the user.
+- Wallet and channel state may be persisted to an [SQLite][sqlite] or [PostgreSQL][postgresql] database, to the filesystem, to a VSS server, or to a custom back-end to be implemented by the user.
 - Gossip data may be sourced via Lightning's peer-to-peer network or the [Rapid Gossip Sync](https://docs.rs/lightning-rapid-gossip-sync/*/lightning_rapid_gossip_sync/) protocol.
 - Entropy for the Lightning and on-chain wallets may be sourced from raw bytes or a [BIP39](https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki) mnemonic. In addition, LDK Node offers the means to generate and persist the entropy bytes to disk.
 
+### Cargo Features
+
+LDK Node's optional dependencies are grouped by the functionality they provide:
+
+| Feature | Functionality |
+| --- | --- |
+| `chain-esplora` | Esplora chain source |
+| `chain-electrum` | Electrum chain source |
+| `chain-bitcoind` | Bitcoin Core RPC and REST chain source |
+| `storage-sqlite` | SQLite storage |
+| `storage-filesystem` | Filesystem storage |
+| `storage-vss` | Versioned Storage Service storage |
+| `storage-postgres` | PostgreSQL storage |
+| `unified-payments` | BIP 21 and human-readable-name payment support |
+| `uniffi` | UniFFI language bindings |
+| `uniffi-default` | The standard language-binding feature set |
+
+The `default` feature set preserves the native Rust API's previous behavior. It enables all three
+chain sources, SQLite, filesystem and VSS storage, and unified payments. PostgreSQL and UniFFI
+remain opt-in. Every build must enable at least one chain source feature.
+
+Disable the default features to select only the functionality and dependencies an application
+needs. For example:
+
+```shell
+cargo build --no-default-features --features chain-esplora,storage-sqlite
+```
+
+`uniffi-default` enables UniFFI, Esplora, Electrum, SQLite, VSS, and unified payments. It excludes
+Bitcoin Core, filesystem storage, and PostgreSQL. Binding users can add any of those features:
+
+```shell
+cargo build --no-default-features --features uniffi-default,chain-bitcoind
+```
+
+Use `uniffi` directly instead of `uniffi-default` to assemble a fully custom binding build. For
+example, a Bitcoin Core and PostgreSQL-only binding build uses:
+
+```shell
+cargo build --no-default-features --features uniffi,chain-bitcoind,storage-postgres
+```
+
+The binding generation scripts use `uniffi-default`. Set `LDK_NODE_EXTRA_FEATURES` to add features
+to their builds:
+
+```shell
+LDK_NODE_EXTRA_FEATURES=chain-bitcoind,storage-postgres \
+	./scripts/uniffi_bindgen_generate_python.sh
+```
+
 ## Compatibility
 
 LDK Node does not provide a stable public API until v1.0. Persisted node state is backwards compatible: newer releases are guaranteed to load state written by older releases. Downgrades are not supported, so state written by a newer release may not load with an older release.