[DOC] Document how to port custom changes from cudf into this project

[jlowe]

One question that came up during review is how does one who's used to working in cudf apply/test their custom changes to cudf against the RAPIDS Accelerator when it's no longer using cudf directly but instead using the spark-rapids-jni artifact. There should be documentation (and ideally scripts if the steps are complicated or hard to remember) on how one can port changes against the cudf repo, either changes to a local cudf repository or a pending cudf PR, into this repo.

[jlowe]

Fixed by #381