add some basic Enum documentation

lib/Enum.pod

@@ -0,0 +1,31 @@
+=begin pod
+
+=head1 Enum
+
+    class Enum does Associative { ... }
+
+An C<Enum> consists of a key and a value. It is the immutable version of
+C<Pair> (which inherits from Enum).
+
+=head2 Methods
+
+=head3 key
+
+    multi method key(Enum:D:)
+
+Returns the key part of the Enum
+
+=head3 value
+
+    multi method value(Enum:D:)
+
+Returns the value part of the Enum
+
+=head3 invert
+
+    multi method invert(Enum:D:) returns Enum:D
+
+Returns a new C<Enum> with the original enum's value as the key, and the
+original enum's key as value.
+
+=end pod