Add contributing guide standards as a separate chapter

[maelle]

via ropensci/opencage#128 (comment) from @dpprdan

In #308 I'll only tackle adding more tips to the existing chapter.

[maelle]

@mpadge assigning you as it's part of the roadmap you wrote in #366 (comment)

[mpadge]

Closing this for now, as it'll all be addressed via #366, and PR #462. Contributing won't be a separate chapter, but i think that #462 makes expectations and requirements sufficiently more explicit.