docs: add note on builtins (#215)

Fixes #146

README.md

@@ -112,6 +112,23 @@ export default {
 };
 ```
 
+## Resolving Built-Ins (like `fs`)
+
+This plugin won't resolve any builtins (e.g. `fs`). If you need to resolve builtins you can install local modules and set `preferBuiltins` to `false`, or install a plugin like [rollup-plugin-node-builtins](https://github.com/calvinmetcalf/rollup-plugin-node-builtins) which provides stubbed versions of these methods.
+
+If you want to silence warnings about builtins, you can add the list of builtins to the `externals` option; like so:
+
+```js
+import resolve from 'rollup-plugin-node-resolve';
+import builtins from 'builtin-modules'
+export default ({
+  input: ...,
+  plugins: [resolve()],
+  externals: builtins,
+  output: ...
+})
+```
+
 
 ## License