Update API-CHANGELOG.md

Author: Bronek

API-CHANGELOG.md

@@ -125,6 +125,13 @@ Currently (prior to the release of 2.0), it is available as a "beta" version, me
 
 Since `api_version` 2 is in "beta", breaking changes to V2 can currently be made at any time.
 
+#### Retired methods
+
+In API version 2, the following methods are no longer available:
+
+- `tx_history` - please use other appropriate method, e.g. `tx` or `account_tx` or `ledger` with `transactions` field set to `true`
+- `ledger_header` - please use `ledger` with `binary` field set to `true`
+
 #### Modifications to account_info response in V2
 
 - `signer_lists` is returned in the root of the response. Previously, in API version 1, it was nested under `account_data`. (https://github.com/XRPLF/rippled/pull/3770)