[doc] add release note for submitpackage

Author: glozow

doc/release-notes-27609.md

@@ -0,0 +1,14 @@
+- A new RPC, `submitpackage`, has been added. It can be used to submit a list of raw hex
+  transactions to the mempool to be evaluated as a package using consensus and mempool policy rules.
+These policies include package CPFP, allowing a child with high fees to bump a parent below the
+mempool minimum feerate (but not minimum relay feerate).
+
+    - Warning: successful submission does not mean the transactions will propagate throughout the
+      network, as package relay is not supported.
+
+    - Not all features are available. The package is limited to a child with all of its
+      unconfirmed parents, and no parent may spend the output of another parent.  Also, package
+      RBF is not supported. Refer to doc/policy/packages.md for more details on package policies
+      and limitations.
+
+    - This RPC is experimental. Its interface may change.