Add a changelog, fixes JedWatson#40

Author: JedWatson

HISTORY.md

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+# React-CodeMirror
+
+## Unreleased
+
+* changed; `path` prop has been changed to `name`
+* changed; Updates peerDependency for `react` and `react-dom` to `">=15.5 <16"`
+* fixed; Removed use of string refs and `React.findDOMNode`
+* fixed; Resolved React v15.5 warnings
+* fixed; Only updates the CodeMirror value if `props.value` has changed
+* fixed; Only updates CodeMirror option values if they have changed
+* added; `onCursorActivity` prop
+* added; `autoFocus` prop
+
+## Previous
+
+See commit history for earlier changes