chore: add ROADMAP document describing the purpose of the package (#505)
chore: add ROADMAP document describing the purpose of the package (#505)

* doc: add ROADMAP document describing the purpose of the package

* additional thoughts

Co-authored-by: Lo Ferris <03abe5595587ef5033d3999f81c048b015879b79@users.noreply.github.com>
Co-authored-by: Anthonios Partheniou <50de7473c4bcd6fb06c46bebfdc91e30df56824a@google.com>
googleapis/python-bigquery-pandas,googleapis/python-bigquery-pandas