Add document to capture breaking changes

docs/breaking-changes.md

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+# Breaking Changes
+
+This document is supposed to capture cases where we'd like to deviate in
+behavior and/or API surface from our goal of being 100% compatible with .NET
+Framework, Mono, and Xamarin.
+
+Please note that this doesn't imply that we'll **actually** break compatibility.
+However, maintainin this list is helpful for making educated decisions around
+the degree to which compatibility can -- and should -- be achieved.
+
+## Binary Serialization
+
+* Right now, our contracts imply a large chunks of all the framework types are
+  marked as serializable
+* Changing the contracts isn't helping because it's about whether the runtime
+  view implements
+
+### Investigation
+
+* Review list of types in the .NET Core and see what the current and future
+  impact of serialization would be
+* Find types that implement `ISerializable` and/or are marked `[Serializable]`
+  and include the list of framework types that reference