add upgrade guide (#829)

docs/.vuepress/config.js

@@ -78,6 +78,7 @@ module.exports = {
             ['guide/options', 'Dropdown Options'],
             ['guide/values', 'Selecting Values'],
             ['guide/localization', 'Localization'],
+            ['guide/upgrading', 'Upgrading 2.x to 3.x'],
           ],
         },
         {

docs/guide/upgrading.md

@@ -0,0 +1,90 @@
+## Removed Function Callbacks
+
+Three function callbacks have been removed in favor of using events. 
+
+- `onChange`
+- `onInput`
+- `onSearch`
+
+While this is a breaking change, the change in your application should be as simple as swapping the
+prop you were using for an event.
+
+### `onChange` & `onInput`
+
+In v2.x, Overwriting `onChange` in an application was more likely to break vue-select's internals 
+and cause issues. The `input` event provides identical functionality and can be swapped out in your 
+application. 
+
+```html
+<!-- version 2: -->
+<v-select :on-change="doSomething" />
+
+<!-- version 3: -->
+<v-select @input="doSomething" />
+```
+
+Additionally, the `change` event has been removed. This event was redundant, `input` should be used 
+instead.
+
+```html
+<!-- version 2: -->
+<v-select @change="doSomething" />
+
+<!-- version 3: -->
+<v-select @input="doSomething" />
+```
+
+### `onSearch`
+
+The `onSearch` prop was removed for the same reason as `onChange` and `onInput`. The `search` event
+has always provided the same parameters and can be used in it's place.
+
+```html
+<!-- version 2: -->
+<v-select :on-search="doSomeAjax" />
+
+<!-- version 3: -->
+<v-select @search="doSomeAjax" />
+```
+
+## Separated CSS
+
+CSS was removed from the JS bundle in favor of a separate CSS file to support SSR and easier
+customization.
+
+  ```js
+  @import vSelect from 'vue-select`;
+  @import 'vue-select/dist/vue-select.css';
+  ```
+
+## New Class Selector Prefix
+
+In order to avoid CSS collisions and allow for low specificity overrides of CSS, all classes have 
+been renamed to include the `vs__` prefix. The full list of renamed classes are listed below:
+
+| original | renamed |
+| ------- | --------- |
+| `.open-indicator` | `.vs__open-indicator` |
+| `.dropdown-toggle` | `.vs__dropdown-toggle` |
+| `.dropdown-menu` | `.vs__dropdown-menu` |
+| `.clear` | `.vs__clear` |
+| `.selected-tag` | `.vs__selected` |
+| `.no-options` | `.vs__no-options` |
+| `.spinner` | `.vs__spinner` |
+| `.close` | `.vs__deselect` |
+| `.active` | `.vs__active` |
+
+## Internal State
+
+**The changes listed below are very unlikely to break your apps** unless you've been hooking into 
+vue-select internal values. [#781](https://github.com/sagalbot/vue-select/pull/781) 
+(thanks [@owenconti!](https://github.com/owenconti)) introduced a number of optimizations to the 
+way that the component handles internal state.
+
+- `value`: the `value` prop is undefined by default. vue-select no longer maintains an internal `mutableValue` state when a `value` prop has been passed. When `:value` or `v-model` is not used, vue-select will maintain internal state using the `_value` property.
+- `mutableOptions` has been removed in favor of an `optionList` computed property.
+
+## Misc
+
+- `fade` transition renamed to `vs__fade`
+- Removed `a` element that was serving as the click handler within dropdown options