Added pick including all keys example

pkg/yqlib/doc/operators/pick.md

@@ -26,6 +26,30 @@ myMap:
   cat: meow
 ```
 
+## Pick keys from map, included all the keys
+We create a map of the picked keys plus all the current keys, and run that through unique
+
+Given a sample.yml file of:
+```yaml
+myMap:
+  cat: meow
+  dog: bark
+  thing: hamster
+  hamster: squeak
+```
+then
+```bash
+yq '.myMap |= pick( (["thing"] + keys) | unique)' sample.yml
+```
+will output
+```yaml
+myMap:
+  thing: hamster
+  cat: meow
+  dog: bark
+  hamster: squeak
+```
+
 ## Pick indices from array
 Note that the order of the indices matches the pick order and non existent indices are skipped.
 

pkg/yqlib/operator_pick_test.go

@@ -14,6 +14,15 @@ var pickOperatorScenarios = []expressionScenario{
 			"D0, P[], (!!map)::myMap: {hamster: squeak, cat: meow}\n",
 		},
 	},
+	{
+		description:    "Pick keys from map, included all the keys",
+		subdescription: "We create a map of the picked keys plus all the current keys, and run that through unique",
+		document:       "myMap: {cat: meow, dog: bark, thing: hamster, hamster: squeak}\n",
+		expression:     `.myMap |= pick( (["thing"] + keys) | unique)`,
+		expected: []string{
+			"D0, P[], (!!map)::myMap: {thing: hamster, cat: meow, dog: bark, hamster: squeak}\n",
+		},
+	},
 	{
 		description: "Pick splat",
 		skipDoc:     true,