Prepare change log for v0.3.1.

Author: paurkedal

CHANGES.md

@@ -1,5 +1,13 @@
 # Changes
 
+## v0.3.1 - 2017-08-21
+
+- Fix accidental shadowing of open from another interface-less module using
+  `ppx_regexp`.
+- Support binding of group 0 and the universal pattern.
+- Switch to `ppx_tools_versioned`. This provides support for 4.02.3 in the
+  main branch.
+
 ## v0.3.0 - 2017-06-04
 
 - Initial release for OCaml 4.03.0 and 4.04.1.