changelog: Add entry about C++ evmc::host
## [6.2.0] - Unreleased
+- Added: [[#217](https://github.com/ethereum/evmc/pull/217)]
+ The `evmc::host` C++ wrapper for EVMC host context and interface
+ to be used by VM implementations.
- Deprecated: [[#196](https://github.com/ethereum/evmc/pull/196)]
The `EVMC_CONSTANTINOPLE2` revision name is deprecated, replaced with `EVMC_PETERSBURG`.
