Update changelog.md

changelog.md

 Notice: All 1.7.x changes are present in 2.0.x aswell
 
 # next
-- [FEATURE] Hooks can now return promises
+- [FEATURE] You can now return a promise to a hook rather than use a callback
 - [FEATURE] There is now basic support for assigning a field name to an attribute `name: {type: DataTypes.STRING, field: 'full_name'}`
 - [FEATURE] It's now possible to add multiple relations to a hasMany association, modelInstance.addRelations([otherInstanceA, otherInstanceB])
 - [FEATURE] `define()` stores models in `sequelize.models` Object e.g. `sequelize.models.MyModel`