add achievement command to docs/topgg.md
* `/randommc` *shows a random Minecraft server if you want a new place to explore*
* `/mcimage` *turns whatever image you upload into Minecraft blocks / pixel art*
* `/stealskin <username>` *fetches the skin of a Minecraft Java Edition player*
+* `/achievement <text>` *generates a Minecraft achievement from the given text*
* `/buildidea` *sends a random build idea for if you're bored and need inspiration*
* `/mccolors` *shows the Minecraft colors and how to use them in the ingame chat*
* `/nametouuid <username>` *turns a MC Java Edition username into a uuid*
