[Core] Added changelog (#222)

Author: Nyholm

CHANGELOG.md

@@ -6,11 +6,14 @@
 
 - Class `AsuncAws\Core\Credentials\NullProvider`
 - Methods `AwsClient::cloudFormation()`, `AwsClient::lambda()`, `AwsClient::sns()`
+- Protected methods `Result::registerPrefetch()` and `Result::unregisterPrefetch()`
+- Timeout parameter to `InstanceProvider::__construct()`
 
 ### Changed
 
 - Removed `AwsClient` and replaced it with `AwsClientFactory`
 - Class `AsuncAws\Core\Signers\Request` is marked as internal
+- Make sure behavior of calling `Result::resolve()` is consistent.
 
 ## 0.1.0