Added method sum() to Range pod

Author: ahalbert

doc/Type/Range.pod6

@@ -181,4 +181,12 @@ by not actually generating the list if it is not necessary.
 Performs the same function as C<Range.list.roll>, but attempts to optimize
 by not actually generating the list if it is not necessary.
 
+=head2 method sum
+
+    multi method sum() returns Numeric:D
+
+Returns the sum of all elements in the Range. Throws X::Str::Numeric if an element can not be coerced into Numeric.    
+
+    (1..10).sum #55
+
 =end pod