Add release notes for `v0.4.0`

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+---
+title: "Release v0.4.0"
+description: "Announcing the v0.4.0 release of BDK"
+author: "Alekos Filini"
+date: "2021-02-17"
+tags: ["rust", "release"]
+hidden: true
+draft: false
+---
+
+A new release of BDK is out: the [`v0.4.0`] release brings updated dependencies, more sanity checks and an overhauled API to build transactions.
+
+You can find the full [v0.4.0 changelog][changelog] on GitHub.
+
+## What's new in v0.4.0
+
+Below are some highlights of the new improved APIs coming with this release:
+
+### A new API to build transaction
+
+The star of this release is the new API designed and implemented by [@llfourn] that brings much more flexibility to the way we create transactions: originally the process of making a transaction involved the creation of a `TxBuilder` which was used
+to configure *how* the wallet should build the transaction. Things like which outputs to create, what `nLockTime` to use, which UTXOs to spend, and much more.
+
+Once fully configured, this builder was then given to the `Wallet` itself in a `Wallet::create_tx()` or `Wallet::bump_fee()` call: the `Wallet` would try to follow the *instructions* given by the builder, but in
+case of conflicting or straight-up wrong options it would have to fail and force the user to start over.
+
+The new API maintains the concept of a *builder*, but it changes the way it's created so that it always contains a reference to the main `Wallet` instance. What this means is that most checks can now be performed right
+when something is added to the builder, not at the end, allowing the user to recover from errors instead of having to start over.
+
+This also opens the door to even more improvements and additions, such as a way to [spend foreign utxos][foreign_utxo] in a transaction, or even a way to [bump the fees of multiple transactions at once][bump_fee_batched] by batching them together, which
+saves a bit of space and money.
+
+```rust
+let send_to = wallet.get_new_address()?;
+let (psbt, details) = {
+    let mut builder = wallet.build_tx();
+    builder
+        .add_recipient(send_to.script_pubkey(), 50_000)
+        .enable_rbf()
+        .do_not_spend_change()
+        .fee_rate(FeeRate::from_sat_per_vb(5.0));
+    builder.finish()?
+};
+```
+
+### Upgraded dependencies
+
+This release also brings many updates to our dependencies, including:
+
+- `bitcoin` to `v0.26`
+- `miniscript` to `v5.1`
+- `electrum-client` to `v0.6`
+- `tokio` to `v1`
+- `reqwest` to `v0.11`
+- `cc` to `>= v1.0.64`
+
+### Compact Filters example
+
+Thanks to the upgrade to `bitcoin v0.26` all the issues related to new networking messages in the P2P Bitcoin network have been fixed, which means that we can finally use our (experimental) compact filters `Blockchain` with
+standard Bitcoin Core 0.21 full nodes.
+
+The following example has also been added to the repository and can be run with `cargo run --features=compact_filters --example compact_filters_balance`.
+
+```rust
+/// This will return wallet balance using compact filters
+/// Requires a synced local bitcoin node 0.21 running on testnet with blockfilterindex=1 and peerblockfilters=1
+fn main() -> Result<(), CompactFiltersError> {
+    env_logger::init();
+    info!("start");
+
+    let num_threads = 4;
+    let mempool = Arc::new(Mempool::default());
+    let peers = (0..num_threads)
+        .map(|_| Peer::connect("localhost:18333", Arc::clone(&mempool), Network::Testnet))
+        .collect::<Result<_, _>>()?;
+    let blockchain = CompactFiltersBlockchain::new(peers, "./wallet-filters", Some(500_000))?;
+    info!("done {:?}", blockchain);
+    let descriptor = "wpkh(tpubD6NzVbkrYhZ4X2yy78HWrr1M9NT8dKeWfzNiQqDdMqqa9UmmGztGGz6TaLFGsLfdft5iu32gxq1T4eMNxExNNWzVCpf9Y6JZi5TnqoC9wJq/*)";
+
+    let database = MemoryDatabase::default();
+    let wallet =
+        Arc::new(Wallet::new(descriptor, None, Network::Testnet, database, blockchain).unwrap());
+    wallet.sync(noop_progress(), None).unwrap();
+    info!("balance: {}", wallet.get_balance()?);
+    Ok(())
+}
+```
+
+## Contributors
+
+A huge thanks to everybody who contributed to this new release with suggestions, pull requests and bug reports.
+
+# TODO: update numbers after release
+# TODO: update changelog URL
+
+Since the `v0.3.0` release around a month ago, we've had `59` new commits made by `8` different contributors for a total of `2462` additions and `1988` deletions. Here's the [full diff][gh_diff].
+
+A special thanks to the new contributor for this release:
+
+- [@luckysori][@luckysori] - Lucas Soriano
+
+[changelog]: https://github.com/bitcoindevkit/bdk/blob/75669049268bbc294564f8c6e0528e07a546258f/CHANGELOG.md#v030---v020
+[foreign_utxo]: https://github.com/bitcoindevkit/bdk/pull/279
+[bump_fee_batched]: https://github.com/bitcoindevkit/bdk/issues/280
+[gh_diff]: https://github.com/bitcoindevkit/bdk/compare/v0.3.0...v0.4.0
+
+[`v0.4.0`]: https://crates.io/crates/bdk/0.4.0
+
+[@luckysori]: https://github.com/luckysori
+[@llfourn]: https://github.com/llfourn