Update version number in changelog
# CHANGELOG
-## 0.6.0 (unreleased)
+## 0.6.0 (2019-08-12)
- Updated `graphql-core-next` to 1.1.1 which has feature parity with GraphQL.js 14.4.0.
- Added basic extensions system to the `ariadne.graphql.graphql`. Currently only available in the `ariadne.asgi.GraphQL` app.
