Add a changelog file

Following guidelines from http://keepachangelog.com

CHANGELOG.md

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+# Change Log
+All notable changes to this project will be documented in this file.
+This project adheres to [Semantic Versioning](http://semver.org/).
+
+## [0.10] - 2016-06-28
+### Added
+- Add a cookie session storage mechanism, as an alternative to the in-memory session pool.
+Sessions stored on the client include a secure digest of the content to prevent against session forgery. ([#4])
+- Cookie session storage supports secret key rotation. ([#48]) 
+- Add an hex decoder that decodes hexadecimal representations to their bytes form. 
+`HexEncoder` does encoding and decoding to/from hex representations. ([#47])
+- Add a flash hash as a way of passing messages through redirection. 
+Anything in the flash is exposed to the very next request and then cleared out. ([#44])
+- Session pool can now renew a session id whenever the session changes. 
+This helps prevent from session fixation attacks. ([#41])
+
+### Changed
+- `SessionIdentifierPolicy` now receives the session data to support more complex use cases of session id generation
+
+### Fixed
+- `ConditionalGet` middleware no longer throws an exception when _Modified-Since_ header has unsupported format
+
+[0.10]: https://github.com/testinfected/molecule/compare/v0.10...v0.9.1
+
+[#48]: https://github.com/testinfected/molecule/issues/48
+[#47]: https://github.com/testinfected/molecule/issues/47
+[#44]: https://github.com/testinfected/molecule/issues/44
+[#41]: https://github.com/testinfected/molecule/issues/41
+[#4]: https://github.com/testinfected/molecule/issues/4