add instructions on how to use this wrapper with Bitbucket.org

[jlouros]

issue reported by @jhallam3 and imported from https://github.com/jlouros/BitBucketServerCSharp/issues/1

This project was developed uniquely with BitBucket Server in mind (on-premises version of BitBucket). Even though the API might be similar, the current implementation would have to be thoroughly tested. To begin with we need to ensure how to handle authentication.

Additional research required.