Write an RFC for First-class Perforce integration

Created by: unknwon

Based on the investigation in https://github.com/sourcegraph/customer/issues/148.

RFC: https://docs.google.com/document/d/1r6ngZNOENVikcEzMU4v8b3TNT2TQydzILVDRqmJuzqY/edit#heading=h.trqab8y0kufp