Added design-decisions.md

docs/internals/design-decisions.md

+Design Decisions
+================
+
+This document lists the design decisions that we have made after extensive discussions. Unless there are very strong
+reasons, these decisions should be kept for consistency. Any change to these decisions should get agreement among
+the core developers.
+
+1. **[When to support path aliases](https://github.com/yiisoft/yii2/pull/3079#issuecomment-40312268)**
+   we should support path alias for properties that are configurable because using path aliases in configurations 
+   are very convenient. In other cases, we should restrict the support for path aliases.