[DOC] base_gengo: add changelog

addons/base_gengo/doc/changelog.rst

+========================
+``base_gengo`` changelog
+========================
+
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+saas-5
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+
+  - Library update: ``mygengo`` (https://pypi.python.org/pypi/mygengo/1.3.3) was outdated and has been replaced by ``gengo`` (https://pypi.python.org/pypi/gengo).