add changes.md to root of project

Changes.md

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+## v.Next
+* Changed BotContext to no longer be dynamic, and add IBotContext.Get<>() IBotContext.Set() for middleware to register 
+* renamed UserStateManagerMiddleware, ConversationManagerMiddleware to UserState<> and ConversationState<> 
+consolidating using generics with POCO objects and shared base class
+* Added BotContextWrapper so developer can easily create custom BotContext with their own helpers/methods
+
+
+## 4.0.0-alpha201802027a
+* Milan release 
+
+