📝 Add Guide for Node.js users

Author: lambdalisue

README.md

@@ -136,6 +136,24 @@ const a: unknown = "Hello";
 const _: string = maybe(a, is.String) ?? "default value";
 ```
 
+## Node.js (npm)
+
+To use `unknownutil` in [Node.js][nodejs], install `unknownutil` like
+
+```console
+npm i unknownutil
+```
+
+Then import `is`, `assert`, `ensure`, and `maybe` like:
+
+```typescript, ignore
+import { assert, ensure, is, maybe } from "unknownutil";
+
+// ...
+```
+
+[nodejs]: https://nodejs.org/
+
 ## Migration
 
 See [GitHub Wiki](https://github.com/lambdalisue/deno-unknownutil/wiki) for