Added lume.invert()

README.md

@@ -115,6 +115,13 @@ an error is raised,
 lume.reduce({1, 2, 3}, function(a, b) return a + b end) -- Returns 6
 ```
 
+### lume.invert(t)
+Returns a copy of the table where the keys have become the values and the
+values the keys.
+```lua
+lume.invert({a = "x", b = "y"}) -- returns {x = "a", y = "b"}
+```
+
 ### lume.set(t [, retainkeys])
 Returns a copy of the `t` table with all the duplicate values removed. If
 `retainkeys` is true the table is not treated as an array and retains its