docs: note about existing content at same path on different branches

vintro

2023-08-14 f1c9ca495e450ecb62dade70c4b60d86e106f79c

 README.md

@@ -9,6 +9,6 @@

**If you are looking for Quartz v3, you can find it on the [`hugo` branch](https://github.com/jackyzha0/quartz/tree/hugo).**

🔗 Read the documentation and setup instructions: https://quartz.jzhao.xyz/
🔗 Read the documentation and setup instructions: https://four.quartz.jzhao.xyz/

[Join the Discord Community](https://discord.gg/cRFFHYye7t)