Changelog: Add entry about Constantinople2
The **Istanbul** EVM revision.
- Added: [[#182](https://github.com/ethereum/evmc/pull/182)]
The `is_zero()` C++ helper for basic data types.
+- Added: [[#186](https://github.com/ethereum/evmc/pull/186)]
+ Reserved the post-Constantinople EVM revision number.
- Deprecated: [[#184](https://github.com/ethereum/evmc/pull/184)]
The `EVMC_LATEST_REVISION` is deprecated, replaced with `EVMC_MAX_REVISION`.
