Add more documentation

README.md

···
 ## Containers
 - [discordnotification](./discordnotification/): Send a message to a Discord channel when the build is a success/fail via Discord Webhook
 - [itchci](./itchci/): A container with itch.io butler already installed
+- [steamci](./steamci/): A plugin to help you publish your game on Steam from CI
 - [godotci](./godotci/): Contains everything you need to export a Godot game (Godot and export templates in the good folder) for Windows/Linux/Web/Android

steamci/README.md

···
 # SteamCI
+```
+ghcr.io/bigaston/steamci
+```
 
 This is a all contained Woodpecker CI plugins that will generate the .vdf of Steam for you, and then upload your game on the specified depot.