Added prepend array example

pkg/yqlib/doc/operators/add.md

@@ -86,6 +86,23 @@ will output
 a: ['dog', 'cat']
 ```
 
+## Prepend to existing array
+Given a sample.yml file of:
+```yaml
+a:
+  - dog
+```
+then
+```bash
+yq '.a = ["cat"] + .a' sample.yml
+```
+will output
+```yaml
+a:
+  - cat
+  - dog
+```
+
 ## Add new object to array
 Given a sample.yml file of:
 ```yaml

pkg/yqlib/operator_add_test.go

@@ -101,6 +101,14 @@ var addOperatorScenarios = []expressionScenario{
 			"D0, P[], (doc)::a: ['dog', 'cat']\n",
 		},
 	},
+	{
+		description: "Prepend to existing array",
+		document:    `a: [dog]`,
+		expression:  `.a = ["cat"] + .a`,
+		expected: []string{
+			"D0, P[], (doc)::a: [cat, dog]\n",
+		},
+	},
 	{
 		skipDoc:        true,
 		description:    "Concatenate to existing array",