doc: add release notes

Co-Authored-By: tdb3 <106488469+tdb3@users.noreply.github.com>

doc/release-notes-31583.md

@@ -0,0 +1,9 @@
+Updated RPCs
+---
+- `getmininginfo` now returns `nBits` and the current target in the `target` field. It also returns a `next` object which specifies the `height`, `nBits`, `difficulty`, and `target` for the next block.
+- `getblock` and `getblockheader` now return the current target in the `target` field
+- `getblockchaininfo` and `getchainstates` now return `nBits` and the current target in the `target` field
+
+REST interface
+---
+- `GET /rest/block/<BLOCK-HASH>.json` and `GET /rest/headers/<BLOCK-HASH>.json` now return the current target in the `target` field