add change log

Author: thomaszurkan-optimizely

CHANGELOG.md

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+# Optimizely Swift SDK Changelog
+
+## 3.1.0-RC
+Date
+
+This the initial release of the Swift SDK. It is a pure Swift implementation take complete advantage of language features and performance.  It supports objective-C as well through objective-c only API. It uses Swift 4.2 but we will be upgrading to 5.0 soon.
+
+This modernized version of the SDK is meant to replace the Objective-C SDK.
+
+### New Features
+* No new features for this release. It supports all of the 3.1 features in all of Optimizély's SDKs.
+
+### Bug Fixes:
+* Change all instances of attributes and eventTags from NSDictionary<NSString*,NSObject*> to NSDictionary<NSString*,id>
+
+### Breaking Changes
+