Create BackwardCompatibility.md

.github/BackwardCompatibility.md

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+# Breaking Change Definition
+
+Breaking changes in Azure SDK for .NET are defined as follows:
+
+## Generated classes
+
+- The class is removed
+- The class is renamed
+- The class no longer extends another class
+
+## Properties of a generated class
+
+- A property is removed
+- A property is renamed
+- A property has its type changed
+
+## Methods of a generated class
+- Methods should not be removed
+- Methods should not be renamed
+- Methods should not have their return type changed
+
+## Parameters of methods in a generated class
+- Parameters should not be removed
+- Parameters should not have their type changed
+- Parameters should preserve their ordering, including when a parameter is added
+- Parameters should keep their default value, if they are assigned one