types: improve type exports

packages/runtime-core/README.md

@@ -2,14 +2,25 @@
 
 > This package is published only for typing and building custom renderers. It is NOT meant to be used in applications.
 
-``` ts
-import { createRenderer, h } from '@vue/runtime-core'
+For full exposed APIs, see `src/index.ts`. You can also run `yarn build runtime-core --types` from repo root, which will generate an API report at `temp/runtime-core.api.md`.
 
-const { render } = createRenderer({
-  nodeOps,
-  patchData,
-  teardownVNode
+## Building a Custom Renderer
+
+``` ts
+import { createRenderer, createAppAPI } from '@vue/runtime-core'
+
+// low-level render method
+export const render = createRenderer({
+  pathcProp,
+  insert,
+  remove,
+  createElement,
+  // ...
 })
 
-render(h('div'), container)
+export const createApp = createAppAPI(render)
+
+export * from '@vue/runtime-core'
 ```
+
+See `@vue/runtime-dom` for how a DOM-targeting renderer is implemented.