Convert CHANGES to Markdown and add content

Author: fingolfin

CHANGES

@@ -0,0 +1,25 @@
+# CHANGES to the 'datastructures' GAP package
+
+## X.Y.Z (2019-MM-DD)
+
+  - Require GAP >= 4.10
+
+## 0.2.3 (2018-12-18)
+
+  - Add Slices
+
+## 0.2.2 (2018-08-20)
+
+  - Fix crash when using datastructures in workspaces
+
+## 0.2.1 (2018-07-04)
+
+  - Add documentation for UnionFind
+
+## 0.2.0 (2018-07-04)
+
+## 0.1.3 (2018-06-19)
+
+## 0.1.1 (2017-11-13)
+
+  - initial public release