changelog: Add entry about Host::call() / result fix
# Changelog
+## [6.2.1] - unreleased
+
+- Fixed:
+ [[#256](https://github.com/ethereum/evmc/issues/256),
+ [#257](https://github.com/ethereum/evmc/issues/257)]
+ Disallow implicit conversion from C++ `evmc::result` to `evmc_result`
+ causing unintendent premature releasing of resources.
+
## [6.2.0] - 2019-04-25
- Added: [[#216](https://github.com/ethereum/evmc/pull/216)]
