Update readme.md: add pnpm as install option (pmndrs#1819)

* Update readme.md: add pnpm as install option

* Update readme.md

Co-authored-by: Blazej Sewera <code@sewera.dev>

---------

Co-authored-by: Daishi Kato <dai-shi@users.noreply.github.com>
Co-authored-by: Blazej Sewera <code@sewera.dev>

readme.md

@@ -15,7 +15,7 @@ Don't disregard it because it's cute. It has quite the claws, lots of time was s
 You can try a live demo [here](https://githubbox.com/pmndrs/zustand/tree/main/examples/demo).
 
 ```bash
-npm install zustand # or yarn add zustand
+npm install zustand # or yarn add zustand or pnpm add zustand
 ```
 
 :warning: This readme is written for JavaScript users. If you are a TypeScript user, don't miss [TypeScript Usage](#typescript-usage).