Update readme.md to reflect string/binary handling

[ember-max]

Just wanted to say I almost abandoned using this project, as it appeared that it did not support the new msgpack v5 string/binary distinctions. Just updating the references in readme.md to MSGPACK_OBJECT_RAW to instead reflect that binary is properly parsed as Buffer (and string data as string) might prevent some confusion. Other than that, this appears to be the best msgpack library available for Node.js (both perf and stability), so needless to say I'm glad I decided to try out the binary/string handling before abandoning it. Thanks!