Updated documentation

I saw there was a TODO list with the text

```
Don't re-upload a package if it already exists and has the same hashes.
```
So I decided to implement it myself, but, 

```ruby
    # check if the object already exists  
    if obj.exists?  
       return if (file_md5.to_s == obj.etag.gsub('"', '') or file_md5.to_s == obj.metadata['md5']) 
    end
```
it was already implemented :scream: . So my implementation ended up being a README update. Thanks for the gem BTW it was exactly what I was looking for. This plus a proxy implementing a http basic auth for the private bucket will work perfectly well as private and reliable repo.