Changelog: Add 2 missing entries
## [6.1.0-dev] - unreleased
+- Added: [[#174](https://github.com/ethereum/evmc/pull/174)]
+ The **Istanbul** EVM revision.
+- Added: [[#182](https://github.com/ethereum/evmc/pull/182)]
+ The `is_zero()` C++ helper for basic data types.
- Deprecated: [[#184](https://github.com/ethereum/evmc/pull/184)]
The `EVMC_LATEST_REVISION` is deprecated, replaced with `EVMC_MAX_REVISION`.
