Update CHANGELOG

Author: Stephen Schiffli

CHANGELOG.md

@@ -6,3 +6,17 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased]
 
+## [3.1.3] - 2018-07-16
+### Added
+- Documentation for basic features of this API.
+- Scripts to generate API reference docs.
+- DFU subspec for handing firmware updates.  New cases where bootloader change is required.
+
+### Changed
+- Update Cpp API to [v0.14.0](https://github.com/mbientlab/MetaWear-SDK-Cpp/releases/tag/0.14.0).
+- connectAndSetup will read device info if in MetaBoot mode.
+
+### Fixed
+- connectAndSetup could sometimes hang, canSendWriteWithoutResponse only valid after a first write.
+- UI helpers didn't always invoke delegate methods on Main Thread.
+