Add CHANGELOG.

Author: SlaunchaMan

Diff for: CHANGELOG.md

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+#Change Log
+All notable changes to this project will be documented in this file.
+`AFNetworking` adheres to [Semantic Versioning](http://semver.org/).
+
+--- 
+
+## [3.0.0](https://github.com/AFNetworking/AFNetworkActivityLogger/releases/tag/3.0.0) (03/23/2018)
+Released on Friday, March 23, 2018. All issues associated with this milestone can be found using this [filter](https://github.com/AFNetworking/AFNetworkActivityLogger/issues?q=milestone%3A3.0.0+is%3Aclosed).
+
+#### Changes
+* Add setLogLevel method.
+ * Implemented by Jon Shier in [#43](https://github.com/AFNetworking/AFNetworkActivityLogger/pull/43).