Added changes document for new releases

Author: mbaak

CHANGES.rst

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+=============
+Release notes
+=============
+
+Version 1.0.24, Apr 2021
+------------------------
+* Categorize histogram now handles nones and nans in friendlier way, they are converted to "NaN".
+* make_histogram() now casts spark nulls to nan in case of numeric columns. scala interprets null as 0.
+* SparselyBin histograms did not add up nanflow when added. Now fixed.
+* Added unit test for doing checks on null conversion to nans
+* Use new histogrammar-scala jar files, v1.0.20
+* Added histogrammar-scala v1.0.20 jar files to tests/jars/