Update README.md

README.md

@@ -46,6 +46,7 @@ npm install --save letsencrypt@2.x
 npm install --save le-store-certbot@2.x   # default plugin for accounts, certificates, and keypairs
 npm install --save le-challenge-fs@2.x    # default plugin for challenge handlers
 npm install --save le-acme-core@2.x       # default plugin for ACME spec
+npm install --save le-sni-auto@2.x        # default plugin for SNICallback
 ```
 
 Usage