Added function lume.extend(), updated README and tests

README.md

@@ -98,6 +98,15 @@ local t = { 1, 2, 3 }
 lume.clear(t) -- `t` becomes {}
 ```
 
+### lume.extend(t, ...)
+Copies all the fields from the source tables to the table `t` and returns `t`.
+Tables are iterated in order, so the right-most table overrides existing
+fields.
+```
+local t = { a = 1, b = 2 }
+lume.extend(t, { b = 4, c = 6 }) -- `t` becomes { a = 1, b = 4, c = 6 }
+```
+
 ### lume.shuffle(t)
 Returns a shuffled copy of the array `t`.