Added CHANGELOG.

CHANGELOG.MD

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+# CHANGELOG
+
+## 0.2.0
+
+- Breaking change in API: functions with optionnal time zone
+  argument splitted into 2 different functions. With and without
+  time zone argument.
+- Used format-like function to be able to customize printing.
+- Added printers for basic format.
+- Important bugfix when using mktime.
+
+## 0.1.3
+
+- Printer bugfix.
+
+## 0.1.2
+
+- Printer bugfix.
+
+## 0.1.1
+
+- Fixed OCaml 3.12.1 compatibility.
+
+## 0.1.0
+
+- Permissive module will parse a superset of (a part of)
+  the ISO8601 spec.
+- Printing of date / times / ... with the form defined as
+  the extended format.