Create an importer that given a github repo name imports the basic information using the github API, into Allura. See ForgeImporter's entry points and the
TracProjectImporter classes as similar examples.
Github doesn't have a lot of metadata.
description can go into the "summary" field. homepage can be set. The programming language could be mapped to a programming language trove in Allura. But determining all those mappings (by hand or with some algorithm) is going to be too much work, and not worth it I think.