update readme

Author: JCxYIS

UnityBluetooth-Unity/README.md

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+## Unity-Bluetooth-Unity
+
+## Bluetooth Manager
+You may want to use these functions in order...  
+1. `StartDiscovery`
+2. `GetAvailableDevices`
+3. `Connect`
+4. `Available`+`ReadLine` / `Send`
+5. `Stop`
+
+Utility func  
+- `Toast`
+
+
+## Troubleshoot
+### No device shown after I clicked "Start Discovery" button
+`CHECK PERMISSION! `