Add changelog.

Author: nadiaya

CHANGELOG.rst

@@ -9,6 +9,7 @@ CHANGELOG
 * bug-fix: Local Mode: correctly handle the case where the model output folder doesn't exist yet
 * doc-fix: Fix typos in tensorflow serving documentation
 * doc-fix: Add estimator base classes to API docs
+* feature: Tuner: make input channels optional
 
 1.14.2
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