Add changelog item

Author: gumb0

CHANGELOG.md

@@ -20,6 +20,7 @@
 - Added: [#5701](https://github.com/ethereum/aleth/issues/5701) Outputs ENR text representation in admin.nodeInfo RPC.
 - Added: [#5705](https://github.com/ethereum/aleth/pull/5705) Istanbul support: EIP 1108 Reduce alt_bn128 precompile gas costs.
 - Added: [#5707](https://github.com/ethereum/aleth/pull/5707) Aleth waits for 2 seconds after sending disconnect to peer before closing socket.
+- Added: [#5709](https://github.com/ethereum/aleth/pull/5709) Istanbul support: EIP-2200 Structured Definitions for Net Gas Metering.
 - Added: [#5751](https://github.com/ethereum/aleth/pull/5751) Istanbul support: EIP-152 Add BLAKE2 compression function `F` precompile.
 - Changed: [#5532](https://github.com/ethereum/aleth/pull/5532) The leveldb is upgraded to 1.22. This is breaking change on Windows and the old databases are not compatible.
 - Changed: [#5559](https://github.com/ethereum/aleth/pull/5559) Update peer validation error messages.