docs: added auth yap

Added section on authentication server options for Flipnote Studio.

readme.md

@@ -18,6 +18,17 @@ Install Python dependencies with:
 python3 -m pip install -r requirements.txt
 ```
 
+## Authentication server
+
+Hatena Server also requires a compatible **authentication server** for Flipnote Studio authentication. This server does not provide the external authentication service by itself.
+
+Possible authentication services include:
+
+- **Sudomemo's authentication service** — this may work for personal hardware testing, but its use is **not recommended for emulators or for a publicly released/server deployment**. Always check and follow Sudomemo's current rules and terms before using their infrastructure; do not rely on their service for a deployment they do not permit.
+- **Reximemo's authentication service** — this may be used as an alternative **if it is currently running/available**.
+
+Whichever service you use, configure the server/client setup so Flipnote Studio can complete authentication before attempting to use the Hatena replacement.
+
 ## Run the server
 
 From the repository root: