README: Update change log for DJSON-14
DJSON-13: newline & flush output after pprint

Makes clojure.data.json/pprint behave more like clojure.pprint/pprint,
including the fix for CLJ-987.

clojure.data.json/pprint now flushes the output stream and ensures
that its output ends with a newline. This is a small change in
behavior.