Add merge function doc.

content/en/functions/merge.md

@@ -0,0 +1,40 @@
+---
+title: merge
+description: "`merge` deep merges two maps and returns the resulting map."
+date: 2018-09-14
+categories: [functions]
+menu:
+  docs:
+    parent: "functions"
+keywords: [dictionary]
+signature: ["$params :=  merge $default_params $user_params", $params := $default_params | merge $user_params]
+workson: []
+hugoversion: "0.49"
+relatedfuncs: [append, reflect.IsMap, reflect.IsSlice]
+aliases: []
+---
+
+An example merging two maps.
+
+```go-html-template
+{{ $default_params := dict "color" "blue" "width" "50%" "height" "25%" }}
+{{ $user_params := dict "color" "red" "extra" (dict "duration" 2) }}
+{{ $params := $default_params | merge $user_params }}
+```
+
+Resulting __$params__:
+
+```
+"color": "blue"
+"extra":
+  "duration": 2
+"height": "25%"
+"icon": "mail"
+"width": "50%"
+```
+
+{{% note %}}
+  Regardless of depth, merging only applies to maps. For slices, use [append]({{< ref "functions/append" >}})
+{{% /note %}}
+
+