Add Checksum of ROM file

Hello! Could you add the checksum of a known valid ROM to the README file?

For me, this version works:
```
9EC6D2A5C2FCA81AB86312328779FD042B5F3B920BF65DF9F6B87B376883CB5B
```

If you add it, new users will be able to easily find the correct version. 🙂
