Add release notes for incoming viewing keys

doc/release-notes.md

@@ -4,3 +4,31 @@ release-notes at release time)
 Notable changes
 ===============
 
+Incoming viewing keys
+---------------------
+
+Support for incoming viewing keys, as described in
+[the Zcash protocol spec](https://github.com/zcash/zips/blob/master/protocol/protocol.pdf),
+has been added to the wallet.
+
+Use the `z_exportviewingkey` RPC method to obtain the incoming viewing key for a
+z-address in a node's wallet. For Sprout z-addresses, these always begin with
+"ZiVK" (or "ZiVt" for testnet z-addresses). Use `z_importviewingkey` to import
+these into another node.
+
+A node that possesses an incoming viewing key for a z-address can view all past
+transactions received by that address, as well as all future transactions sent
+to it, by using `z_listreceivedbyaddress`. They cannot spend any funds from the
+address. This is similar to the behaviour of "watch-only" t-addresses.
+
+`z_gettotalbalance` now has an additional boolean parameter for including the
+balance of "watch-only" addresses (both transparent and shielded), which is set
+to `false` by default. `z_getbalance` has also been updated to work with
+watch-only addresses.
+
+- **Caution:** for z-addresses, these balances will **not** be accurate if any
+  funds have been sent from the address. This is because incoming viewing keys
+  cannot detect spends, and so the "balance" is just the sum of all received
+  notes, including ones that have been spent. Some future use-cases for incoming
+  viewing keys will include synchronization data to keep their balances accurate
+  (e.g. [#2542](https://github.com/zcash/zcash/issues/2542)).