Added CHANGELOG.md

Author: Lev Rubel

CHANGELOG.md

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+### 0.2.11
+ * Fixes UJSONResponse problem that comes from latest Starlette (since new FastAPI release depends on it)
+
+### 0.2.10
+###### Support for secrets file as configuration source
+
+ * Dropped Python 3.6 support
+ * Added Python 3.9 support
+ * FastAPI is updated to the latest version in requirements
+ * Fixed serializer base class bug because of latest pydantic update (1.8)