Add CHANGELOG entry about Cable upgrade
Documentation has been extended.
- Improved: [[#59](https://github.com/ethereum/evmc/pull/59)]
Optional Result Storage helper module has been separated.
+- Improved: [[#75](https://github.com/ethereum/evmc/pull/75)]
+ Cable upgraded to 0.2.11.
## [5.0.0] - 2018-08-10
