Update 'Interface Definitions'

Interface-Definitions.md

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+# Syntax
+```
+face ro ReadWriter:Face
+	write
+		> data:{Byte ..}
+		< wrote:Int
+		< err:Error
+	read
+		> into:{Byte ..}
+		< read:Int
+		< err:Error
+```
+
+Interface sections contain a list of behaviors. Behaviors are a name followed by an indented block of inputs and outputs. It is essentially a more stripped down version of function syntax.
+
+# Semantics
+Interfaces in ARF are like interfaces in Go, that is to say, their methods are
+defined and any object that has at least those methods can fulfill that
+interface.
+
+Interfaces are able to inherit other interfaces using type notation. If an interface inherits nothing, it must inherit `Face`, which is a blank interface.