docs(readme): update installation instructions

packages/pnpm/README.md

@@ -40,6 +40,20 @@ why it works fine with the Node.js ecosystem, read this small article: [Flat nod
 
 ## Installation
 
+On macOS, Linux, or Windows Subsystem for Linux:
+
+```
+curl -f https://get.pnpm.io/v6.js | node - add --global pnpm
+```
+
+On Windows (using PowerShell):
+
+```
+(Invoke-WebRequest 'https://get.pnpm.io/v6.js' -UseBasicParsing).Content | node - add --global pnpm
+```
+
+Using npm:
+
 ```
 npm install -g pnpm
 ```