Improve community status (contributing, explicit license file)

[jefferis]

See https://github.com/jefferis/nat/community

usethis::use_gpl3_license()

[jefferis]

Contributing.md could incorporate some ideas from https://github.com/atom/atom/blob/master/CONTRIBUTING.md and also point to https://devguide.ropensci.org/building.html. It would be good to cover issues of package dependencies explicitly both by linking to https://devguide.ropensci.org/building.html#package-dependencies and making specific recommendations.