Add documentation on "Join discord" configuration

README.md

@@ -140,6 +140,7 @@ Add `config.json` file in root directory with following content. You can use `co
 ```javascript
 {
   "title": "Pony Town",
+  "discordLink": "https://discordapp.com/invite/<invite_code>", // optional
   "twitterLink": "https://twitter.com/<twitter_name>", // optional
   "contactEmail": "<your_contact_email>",
   "port": 8090,

config-template.json

@@ -1,5 +1,6 @@
 {
 	"title": "Your Town",
+	"discordLink": "<LINK_TO_DISCORD_INVITE>",
 	"twitterLink": "<LINK_TO_TWITTER>",
 	"contactEmail": "[email protected]",
 	"port": 8090,