add docs

docs/guide/keydown.md

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+### Customizing Keydown Behaviour
+---
+
+Vue Select provides the `map-keydown` Function prop to allow for customizing the components response to 
+keydown events while the input has focus. Here's the default function definition:
+
+```js
+/**
+ * @param map {Object} Mapped keyCode to handlers { <keyCode>: <callback> }
+ * @param vm {Vue/Component} Vue Select instance
+ * @return {Object}
+ */
+(map, vm) => map,
+```
+
+By default, the prop is a no–op returning the same object `map` object it receives. This object
+maps keyCodes to handlers: `{ <keyCode>: <callback> }`. Modifying this object can override default
+functionality, or add handlers for different keys that the component doesn't normally listen for.
+
+### Example: Tag on `comma` and `space`
+
+If I have a taggable input, and I want `comma` or `space` to 'tag' the current option, you could 
+solve that with map-keydown. Since the tab button already creates tags, we can copy the handler
+for keyCode 9.
+
+```vue
+<template>
+  <v-select taggable multiple no-drop :map-keydown="handlers"/>
+</template>
+
+<script>
+export default {
+  methods: {
+    handlers (map, vm) {
+      const createTag = e => {
+        e.preventDefault();
+        vm.typeAheadSelect();
+        vm.searchEl.focus();
+      };
+
+      return {
+        ...map, //  defaults
+        32: createTag,  //  space
+        188: createTag,  //  comma
+      };
+    },
+  },
+};
+</script>
+```
+
+<custom-handlers></custom-handlers>