[skip ci] nested creation changelog entry

changelog.md

+# Next
+- [FEATURE] Highly experimental support for nested creation [#3386](https://github.com/sequelize/sequelize/pull/3386)
+
 # 2.0.4
 - [BUG] Fixed support for 2 x belongsToMany without foreignKey defined and association getter/adder [#3185](https://github.com/sequelize/sequelize/issues/3185)
 - [BUG] No longer throws on `Model.hasHook()` if no hooks are defiend [#3181](https://github.com/sequelize/sequelize/issues/3181)