Update README.md with more notes

@marco-c recently added profiling for the async natives! They appear after the profile for the Java methods and look something like this:

> Async natives profile:
> 3890ms 3 com/ibm/oti/connection/file/FCOutputStream.openOffsetImpl.([BJ)I
> 3884ms 11 com/ibm/oti/connection/file/Connection.isDirectoryImpl.([B)Z
> 2651ms 8 com/ibm/oti/connection/file/Connection.fileSizeImpl.([B)J
> …