Add solution for exercise A4.

exercise_a4/data_cleaning.md

+# Data cleaning
+
+* Day 1: https://www.kaggle.com/jamalsaf/data-cleaning-challenge-handling-missing-values?scriptVersionId=10396551
+* Day 2: https://www.kaggle.com/jamalsaf/data-cleaning-challenge-scale-and-normalize-data?scriptVersionId=10397085
+* Day 3: https://www.kaggle.com/jamalsaf/data-cleaning-challenge-parsing-dates?scriptVersionId=10397914
+* Day 4: https://www.kaggle.com/jamalsaf/data-cleaning-challenge-character-encodings?scriptVersionId=10399844
+* Day 5: https://www.kaggle.com/jamalsaf/data-cleaning-challenge-inconsistent-data-entry?scriptVersionId=10400306
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