Update release notes

doc/release-notes.md

@@ -4,6 +4,33 @@ release-notes at release time)
 Notable changes
 ===============
 
+Faster block validation for Sapling and Orchard transactions
+------------------------------------------------------------
+
+Block validation in `zcashd` is a mostly single-threaded process, due to how the
+chain update logic inherited from Bitcoin Core is written. However, certain more
+computationally intensive checks are performed more efficiently than checking
+everything individually:
+
+- ECDSA signatures on transparent inputs are checked via multithreading.
+- RedPallas signatures on Orchard actions are checked via batch validation.
+
+As of this release, `zcashd` applies these techniques to more Sapling and
+Orchard components:
+
+- RedJubjub signatures on Sapling Spends are checked via batch validation.
+- Groth16 proofs for Sapling Spends and Outputs are checked via batch validation
+  and multithreading.
+- Halo 2 proofs for Orchard Actions are checked via batch validation and
+  multithreading.
+
+This reduces worst-case block validation times for observed historic blocks by
+around 80% on a Ryzen 9 5950X CPU.
+
+The number of threads used for checking Groth16 and Halo 2 proofs (as well as
+for creating them when spending funds) can be set via the `RAYON_NUM_THREADS`
+environment variable.
+
 Option handling
 ---------------
 
@@ -15,6 +42,12 @@ Option handling
   helpful if recipients of transactions are likely to be using legacy wallets
   that have not yet been upgraded to support parsing V5 transactions.
 
+RPC interface
+-------------
+
+- The `getrawtransaction` RPC method now includes details about Orchard actions
+  within transactions.
+
 Deprecated
 ----------