Add changelog.

Author: noplanman

CHANGELOG.md

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+# Changelog
+The format is based on [Keep a Changelog](http://keepachangelog.com/) and this project adheres to [Semantic Versioning](http://semver.org/).
+
+## [0.41.0] - 2017-03-25
+### Added
+- `$showInHelp` attribute for commands, to set if it should be displayed in the `/help` command.
+- Link to new Telegram group: `https://telegram.me/PHP_Telegram_Bot_Support`
+- Introduce change log.
+
+## [0.40.1] - 2017-03-07
+### Fixed
+- Infinite message loop, caused by incorrect Entity variable.
+
+## [0.40.0] - 2017-02-20
+### Added
+- Request limiter for incoming requests.
+### Fixed
+- Faulty formatting in logger.
+
+## [0.39.0] - 2017-01-20
+### Added
+- Newest bot API changes.
+- Allow direct access to PDO object (`DB::getPdo()`).
+- Simple `/debug` command that displays various system information to help debugging.
+- Crontab-friendly script.
+### Changed
+- Botan integration improvements.
+- Make logger more flexible.
+### Fixed
+- Various bugs and recommendations by Scrutinizer.
+
+## [0.38.1] - 2016-12-25
+### Fixed
+- Usage of self-signed certificates in conjunction with the new `allowed_updates` webhook parameter.
+
+## [0.38.0] - 2016-12-25
+### Added
+- New `switch_inline_query_current_chat` option for inline keyboard.
+- Support for `channel_post` and `edited_channel_post`.
+- New alias `deleteWebhook` (for `unsetWebhook`).
+### Changed 
+- Update WebhookInfo entity and `setWebhook` to allow passing of new arguments.
+
+## [0.37.1] - 2016-12-24
+### Fixed
+- Keyboards that are built without using the KeyboardButton objects.
+- Commands that are called via `/command@botname` by correctly passing them the botname.
+
+## [0.37.0] - 2016-12-13
+### Changed
+- Logging improvements to Botan integration.
+### Deprecated
+- Move `hideKeyboard` to `removeKeyboard`.