servant-client-core-0.12: CHANGELOG.md

[The latest version of this document is on GitHub.](https://github.com/haskell-servant/servant/blob/master/servant-client-core/CHANGELOG.md)
[Changelog for `servant` package contains significant entries for all core packages.](https://github.com/haskell-servant/servant/blob/master/servant/CHANGELOG.md)

0.12
----

- First version. Factored out of `servant-client` all the functionality that was
  independent of the `http-client` backend.