Document change to instance ports and password

Add entry to changelog detailing the change to null by default entries
for the ports and password in the instance config, and update the
comment in it to reflect the recent changes.

CHANGELOG.md

@@ -20,6 +20,9 @@ Version 2.0.0
   copied on instance creation.
 - Instance id is no longer derived from the rcon password, instead it's
   generated upon instance creation and stored in the instance config.
+- The game port, RCON port and RCON password instance config entries are are
+  now null by default, indicating that a random one will be generated every
+  time the instance is started.
 
 ### Breaking Changes
 

client.js

@@ -305,7 +305,7 @@ async function createInstance(config, args, instance) {
 		"id": Math.random() * 2 ** 31 | 0,
 		"factorioPort": args.port || process.env.FACTORIOPORT || null,
 		"clientPort": args["rcon-port"] || process.env.RCONPORT || null,
-		"__comment_clientPassword": "This is the rcon password. Its also used for making an instanceID. Make sure its unique and not blank.",
+		"__comment_clientPassword": "This is the rcon password. Will be randomly generated if null.",
 		"clientPassword": args["rcon-password"] || null,
 		"info": {}
 	}