Add CHANGELOG.md

Author: parroty

CHANGELOG.md

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+0.4.0
+------
+#### Enhancements
+* The `use_cassette` with `custom: true` or `:stub` option can now have either string or regexp format (#13).
+    - This item involves json format change. In order to use regexp matching, please wrap the string with "~/" prefix and "/" suffix (ex. "~/regexpstring/")