[docs] Add release notes for burying bip 9 soft fork deployments

doc/release-notes-16060.md

@@ -0,0 +1,15 @@
+Low-level RPC changes
+----------------------
+
+- Soft fork reporting in the `getblockchaininfo` return object has been
+  updated. For full details, see the RPC help text. In summary:
+  - The `bip9_softforks` sub-object is no longer returned
+  - The `softforks` sub-object now returns an object keyed by soft fork name,
+    rather than an array
+  - Each softfork object in the `softforks` object contains a `type` value which
+    is either `buried` (for soft fork deployments where the activation height is
+    hard-coded into the client implementation), or `bip9` (for soft fork deployments
+    where activation is controlled by BIP 9 signaling).
+
+- `getblocktemplate` no longer returns a `rules` array containing `CSV`
+  and `segwit` (the BIP 9 deployments that are currently in active state).