added release-notes

doc/release-notes-5776.md

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+Added RPC
+--------
+
+- `getassetunlockchainlocks` RPC allows to fetch Asset Unlock txs by their withdrawal index. The RPC accepts an array of indexes and returns status for each index.
+The possible outcomes per each index are:
+- "chainlocked": If the Asset Unlock index is mined on a Chainlocked block.
+- "mined": If no Chainlock information is available, and the Asset Unlock index is mined.
+- "mempooled": If the Asset Unlock index is in the mempool.
+- null: If none of the above are valid.
+
+Note: This RPC is whitelisted for the Platform RPC user.