Document conventions for ordering aliases and arguments in contributing.md

**Description of the desired feature**

Most of the PyGMT modules order aliases and arguments alphabetically by the GMT short option. I think it would be helpful to add this convention (if it is decided) to the contributing guidelines under [example code standards](https://github.com/GenericMappingTools/pygmt/blob/master/CONTRIBUTING.md#example-code-standards).

**Are you willing to help implement and maintain this feature?** Yes