add vue-3 specific doc, add setup syntactic templating sugar.

Author: Ronnie Villarini

vue-3.md

@@ -0,0 +1,32 @@
+# This document is for Vue-3 _specific_ syntax
+
+### Setup Template Attribute Syntax
+
+⚠️ This feature is still a WIP, and is an active RFC. [Follow the conversation](https://github.com/vuejs/rfcs/pull/182) to stay up to date with the lifecycle.
+
+This provides a better DX when using the setup function in Vue SFCs.
+Type inference still works, and you don't need to use `defineComponent`..
+It's important to note that this is just **syntactic sugar**, and still get's compiled down to a component
+using `defineComponent` and the `setup` function.
+
+```vue
+<script setup lang="ts">
+import { ref } from 'vue';
+
+export const welcome = ref('Welcome to Vue 3 with TS!');
+</script>
+```
+
+`props` and the `context` object work as well.
+For TS inference, declare them using TS syntax.
+
+```vue
+<script setup="props, ctx" lang="ts">
+import { computed } from 'vue'
+
+declare const props: {
+    name: string
+}
+
+export const welcome = computed(() => `Welcome, ${props.name}!`)
+```