chore(docs): update engine and readme for Nodejs v12 (#375)

Author: kyranet

README.md

@@ -30,8 +30,8 @@ Additionally, it makes full use of ES2017's `async`/`await` functionality for cl
 - Command throttling/cooldowns
 
 ## Installation
-**Node 8.6.0 or newer is required.**  
-`npm install discord.js-commando`
+**Node 12.0.0 or newer is required.**  
+`npm install discord.js@12 discord.js-commando`
 
 ## Documentation
 [View the docs here.](https://discord.js.org/#/docs/commando)  

package.json

@@ -38,6 +38,6 @@
 		"typescript": "^3.6.2"
 	},
 	"engines": {
-		"node": ">=8.6.0"
+		"node": ">=12.0.0"
 	}
 }