Add a couple of usage examples to the README

[bbatsov]

While updating the nREPL docs (I'm collecting a list of libraries that might be useful to nREPL tool authors) I came across https://github.com/dasch/ruby-bencode/#examples and it'd be nice if we had a few such examples in the README as well.

Might be a good idea to add here some versions of the nestring and bencode functions that work on strings as well, so it's easier for end users to understand how the encoding works.

@shen-tian Perhaps you'd like to tackle this?