docs: add release notes

docs/release-notes/release-notes-0.15.2.md

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+# Release Notes
+
+## RPC/REST Server
+
+- A `POST` URL mapping [was added to the REST version of the `QueryRoutes` call
+  (`POST /v1/graph/routes`)](https://github.com/lightningnetwork/lnd/pull/6926)
+  to make it possible to specify `route_hints` over REST.
+
+# Contributors (Alphabetical Order)
+
+* Oliver Gugger