Added XML decoder (#1044)

pkg/yqlib/doc/operators/union.md

@@ -0,0 +1,33 @@
+# Union
+
+This operator is used to combine different results together.
+
+## Combine scalars
+Running
+```bash
+yq eval --null-input '1, true, "cat"'
+```
+will output
+```yaml
+1
+true
+cat
+```
+
+## Combine selected paths
+Given a sample.yml file of:
+```yaml
+a: fieldA
+b: fieldB
+c: fieldC
+```
+then
+```bash
+yq eval '.a, .c' sample.yml
+```
+will output
+```yaml
+fieldA
+fieldC
+```
+