Add changelog entry for single_attestation event

Author: nflaig

CHANGES.md

@@ -16,6 +16,7 @@ There are likely to be descriptions etc outside of the list below, but new query
 | [#448](https://github.com/ethereum/beacon-APIs/pull/448) `POST /eth/v2/beacon/pool/attester_slashings` added        |                                                  |                                                  |                                                    |                                                 |                                           |
 | [#452](https://github.com/ethereum/beacon-APIs/pull/452) `POST /eth/v1/beacon/states/{state_id}/validator_identities` added |                                          |                                                  |                                                    |                                                 |                                           |
 | [#456](https://github.com/ethereum/beacon-APIs/pull/456) `POST /eth/v2/validator/aggregate_and_proofs` added        |                                                  |                                                  |                                                    |                                                 |                                           |
+| [#472](https://github.com/ethereum/beacon-APIs/pull/472) `single_attestation EVENT` added        |                                                  |                                                  |                                                    |                                                 |                                           |
 
 
 The Following are no longer in the Standard API, removed since the latest version.