Add changelog for beta 2 (#140)

Author: nmuesch

CHANGELOG.md

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+## 1.0.0b2 / 2020-11-05
+
+* [Added] Add missing synthetics step types. See [#129](https://github.com/DataDog/datadog-api-client-python/pull/129).
+* [Added] Add include_tags in logs archives. See [#126](https://github.com/DataDog/datadog-api-client-python/pull/126).
+* [Changed] Change teams and services objects names to be incident specific. See [#134](https://github.com/DataDog/datadog-api-client-python/pull/134).
+* [Removed] Remove `require_full_window` client default value for monitors. See [#135](https://github.com/DataDog/datadog-api-client-python/pull/135).
+
+## 1.0.0b1 / 2020-10-21
+
+* Initial beta release of this client