Added basic documentation for select widget (#806)

docs/widgets.md

@@ -22,6 +22,23 @@ Widgets define the data type and interface for entry fields. Netlify CMS comes w
 
 We’re always adding new widgets, and you can also [create your own](/docs/extending)!
 
+### Select Widget
+
+The select widget allows you to pick a string value from a drop down menu
+
+```yaml
+collections:
+  - name: posts
+    label: Post
+    folder: "_posts"
+    slug: "{{year}}-{{month}}-{{day}}-{{slug}}"
+    create: true
+    fields:
+      - {label: Title, name: title, widget: string, tagname: h1}
+      - {label: Body, name: body, widget: markdown}
+      - {label: Align Content, name: align, widget: select, options: ['left', 'center', 'right']}
+```
+
 ### List Widget
 
 The list widget allows you to map a user-provided string with a comma delimiter into a list. Consider the following example that also demonstrates how to set default values: