docs: add explicit install command to README.md (#84)

## PR Checklist

- [x] Addresses an existing open issue: fixes #83
- [x] That issue was marked as [`status: accepting
prs`](https://github.com/dmnd/dedent/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [x] Steps in
[CONTRIBUTING.md](https://github.com/dmnd/dedent/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

Adds an explicit `npm install dedent` code block to the top of the
README.md

Author: JoshuaKGoldberg

README.md

@@ -27,6 +27,10 @@
 
 ## Usage
 
+```shell
+npm i dedent
+```
+
 ```js
 import dedent from "dedent";