%o is not documented in Readme.md

'%o' is not documented. Or even better would be to not to pass arguments to `util.inspect` directly, but pass them through own formatter depending on type of argument, if it is an Object or Array then format using `formatter.o`:

```
debug('myObj=', myObj); // doesn't work because '%o' is not documented
debug('myObj=%o', myObj); // works because '%o' is now documented and people knows about it
```
