create selectable docs (#996)

docs/guide/selectable.md

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+## Selectable Prop <Badge text="v3.3.0+" />
+
+The `selectable` prop determines if an option is selectable or not. If `selectable` returns false
+for a given option, it will be displayed with a `vs__dropdown-option--disabled` class. The option
+will be disabled and unable to be selected. 
+
+```js
+selectable: {
+  type: Function,
+  /**
+   * @param {Object|String} option
+   * @return {boolean}
+   */
+  default: option => true,
+},
+```
+
+### Example
+
+Here `selectable` is used to prevent books by a certain author from being chosen. In this case,
+the options passed to the component are objects: 
+
+```json
+{ 
+    title: "Right Ho Jeeves", 
+    author: { firstName: "P.D", lastName: "Woodhouse" }, 
+}
+```
+
+This object will be passed to `selectable`, so we can check if the author should be selectable or not.
+
+<UnselectableExample />
+
+<<< @/.vuepress/components/UnselectableExample.vue{6}
+
+## Limiting the Number of Selections
+
+`selectable` can also be used a bit more creatively to limit the number selections that can be made 
+within the component. In this case, the user can select any author, but may only select a maximum
+of three books.
+
+<LimitSelectionQuantity />
+
+<<< @/.vuepress/components/LimitSelectionQuantity.vue{8}