Added XML decoder (#1044)

pkg/yqlib/doc/operators/headers/add.md

@@ -0,0 +1,8 @@
+# Add
+
+Add behaves differently according to the type of the LHS:
+* arrays: concatenate
+* number scalars: arithmetic addition
+* string scalars: concatenate
+
+Use `+=` as append assign for things like increment. Note that `.a += .x` is equivalent to running `.a = .a + .x`.