docs/api-changes/0.55: add file

Author: illwieckz

docs/api-changes/0.55.md

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+# API Breaking changes for the 0.55 development cycle
+
+## 0.55.2+
+
+- **Deprecation:** The `M_PI`, `M_SQRT2` and `M_ROOT3` definitions are deprecated in favor of constants from `common/Math/Constant.h`.
+- **Deprecation:** The `DegToRad()` and `RadToDeg()` macros are deprecated in favor of `Math::DegToRad()` and `Math::RadToDeg()`.