ropensci/dev_guide: First release

2019-01-23, add details about requirements for packges running on all major platforms and added new section to package categories.

2019-01-22, add details to the guide for authors about the development stage at which to submit a package.

2018-12-21, inclusion of an explicity policy for conflict of interest (for reviewers and editors).

2018-12-18, added more guidance for editor on how to look for reviewers.

2018-12-04, onboarding was renamed Software Peer Review.