Add release note for derivation JSON change

doc/manual/rl-next/derivation-json-change.md

@@ -0,0 +1,12 @@
+---
+synopsis: Modify `nix derivation {add,show}` JSON format
+issues: 9866
+prs: 10722
+---
+
+The JSON format for derivations has been slightly revised to better conform to our [JSON guidlines](@docroot@contributing/cli-guideline#returning-future-proof-json).
+In particular, the hash algorithm and content addressing method of content-addresed derivation outputs is now separated into two fields `hashAlgo` and `method`,
+rather than one field with an arcane `:`-separated format.
+
+This JSON format is only used by the experimental `nix derivation` family of commands, at this time.
+Future revisions are expected as the JSON format is still not entirely in compliance even after these changes.