Update the maintainers guide

[seisman]

Description of the problem

Some content of the mainteners guide is outdated. We should update the guide based on our experience of releasing PyGMT v0.1.1.

The updated guide may or may not be included in the v0.1.1 release.

[seisman]

Closed in #446.

[weiji14]

Not sure if this is completely done, for example in https://github.com/GenericMappingTools/pygmt/blob/master/MAINTENANCE.md#continuous-integration, we haven't mentioned anything about using Azure Pipelines.