docs: add pnpm installation

Author: fasenderos

README.md

@@ -34,7 +34,7 @@ Ultra-fast Node.js Order Book written in TypeScript for high-frequency trading (
 Install with npm:
 
 ```sh
-npm install order-book --save
+npm install order-book
 ```
 
 Install with yarn:
@@ -43,6 +43,12 @@ Install with yarn:
 yarn add order-book
 ```
 
+Install with pnpm:
+
+```sh
+pnpm add order-book
+```
+
 ## Usage
 
 To start using order book you need to import `OrderBook` and create new instance: