[all] Add changelog

Author: Samuel Hassine

CHANGELOG.md

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+# Changelog
+
+## Version 1.2.12 (21/07/2019)
+
+#### Enhancements:
+
+- [#12](https://github.com/OpenCTI-Platform/client-python/issues/12) Split STIX2 bundle before sending them in ConnectorHelper
+- [#7](https://github.com/OpenCTI-Platform/client-python/issues/7) Handle STIX 2 indicators import
+
+---
+
+## Version 1.2.9 (11/07/2019)
+
+#### Enhancements:
+
+- [#8](https://github.com/OpenCTI-Platform/client-python/issues/8) Add a new class for connector helper
+
+---
+
+## Version 1.2.4 (05/07/2019)
+
+#### Enhancements:
+
+- [#6](https://github.com/OpenCTI-Platform/client-python/issues/6) Handle observables export as STIX2 indicators
+
+---
+
+## Version 1.2.2 (02/07/2019)
+
+#### Bug Fixes:
+
+- [#3](https://github.com/OpenCTI-Platform/client-python/issues/3) Fix the STIX2 class for a compliant export
+
+---
+
+## Version 1.2.1 (28/06/2019)
+
+#### Enhancements:
+
+- [#2](https://github.com/OpenCTI-Platform/client-python/issues/2) Add a method of health check 
+- [#1](https://github.com/OpenCTI-Platform/client-python/issues/1) Add methods to select observables and their relations