docs: add release notes for v0.15.5-beta

docs/release-notes/release-notes-0.15.5.md

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+# Release Notes
+
+## Bug Fixes
+
+* [A Taproot related key tweak issue was fixed in `btcd` that affected remote
+  signing setups](https://github.com/lightningnetwork/lnd/pull/7130).
+
+# Contributors (Alphabetical Order)
+
+* Oliver Gugger