Added CHANGELOG.md

Author: dcyoung

CHANGELOG.md

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+# Changelog
+
+## 9/7/2017
+- Updated `README`
+- Updated `keywords.txt`
+- Expanded `ScanPacket` struct into full class
+  - `ScanPacket` is now immutable, and its data is accessed through getter methods
+  - Obtaining a reading now requires a separate boolean variable for the success flag
+  - Angle data is efficiently stored in raw 16-bit fixed point value
+  - Getter methods provide an interface to obtain the angle as a raw value or as a float in degrees
+- Removed unused method `angle_raw_to_deg` from `Sweep.h`