docs/release-notes: shift prior 0.15.2 release to 0.15.3

We also add an entry for the _new_ 0.15.2 hot fix release.

Author: Roasbeef

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

@@ -0,0 +1,10 @@
+# Release Notes
+
+## Bug Fixes
+
+* [A wire parsing bug has been fixed that would cause lnd to be unable _decode_
+  certain large blocks](https://github.com/lightningnetwork/lnd/pull/7004)
+
+# Contributors (Alphabetical Order)
+
+* Olaoluwa Osuntokun