Update Discord channel descriptions

CONTRIBUTING.md

@@ -10,13 +10,17 @@ Questions & online chat  [![Discord](https://img.shields.io/discord/539405872346
 -----------------------
 
 We have a [Discord server] to discuss Libplanet.  There are some channels
-for purposes:
+for purposes in the *Libplanet* category:
 
- -  *#libplanet-users*: Chat with game programmers who use Libplanet.
+ -  *#announcement*: Maintainers announce events like our roadmap or local
+    events.
+ -  *#users*: Chat with game programmers who use Libplanet.
     Ask questions to *use* Libplanet for your games.
- -  *#libplanet-dev*: Chat with maintainers and contributors of Libplanet.
+ -  *#development*: Chat with maintainers and contributors of Libplanet.
     Ask questions to *hack* Libplanet and to make a patch for it.
- -  *#libplanet-users-kr*: The same purpose to *#libplanet-users*,
+ -  *#users-ko*: The same purpose to *#users*,
+    except you can speak Korean instead English here.
+ -  *#development-ko*: The same purpose to *#development*,
     except you can speak Korean instead English here.
 
 [Discord server]: https://discord.gg/ue9fgc3