Added sentence about clock syncronization

docs/tutorials/yggdrasil-only-mode.md

@@ -17,3 +17,4 @@ If you use Yggdrasil subnet and want to bind your router to a particular address
 `meshnets.yggaddress=` your local yggdrasil address
 
 Start i2pd, it will also reseed from an yggdrasil reseed server. No clearnet communication is involved.   
+Important: Make sure that the system clock is syncronized.