feat(netlify-cms-widget-select): add support for multiple selection (#1901)

website/content/docs/widgets/select.md

@@ -3,16 +3,17 @@ label: "Select"
 title: select
 ---
 
-The select widget allows you to pick a single string value from a dropdown menu.
+The select widget allows you to pick a string value from a dropdown menu.
 
 - **Name:** `select`
-- **UI:** HTML select input
-- **Data type:** string
+- **UI:** select input
+- **Data type:** string or array
 - **Options:**
   - `default`: accepts a string; defaults to an empty string
   - `options`: (**required**) a list of options for the dropdown menu; can be listed in two ways:
       - string values: the label displayed in the dropdown is the value saved in the file
       - object with `label` and `value` fields: the label displays in the dropdown; the value is saved in the file
+  - `multiple`: accepts a boolean; defaults to `false`
 - **Example** (options as strings):
     ```yaml
     - label: "Align Content"
@@ -30,4 +31,12 @@ The select widget allows you to pick a single string value from a dropdown menu.
         - { label: "Paris", value: "CDG" }
         - { label: "Tokyo", value: "HND" }
     ```
+- **Example** (multiple):
+    ```yaml
+    - label: "Tags"
+      name: "tags"
+      widget: "select"
+      multiple: true
+      options: ["Design", "UX", "Dev"]
+    ```