Add talks/posters/workshops section to AUTHORSHIP.md

**Description of the desired feature**

The authorship policy added in https://github.com/GenericMappingTools/pygmt/pull/726 is really helpful to have for releases and future papers. I found it a bit more confusing how authorship works for conference presentations. For the [AGU 2021 Abstract](https://github.com/GenericMappingTools/pygmt/issues/1395) and the [SciPy 2022 abstract](https://forum.generic-mapping-tools.org/t/pygmt-at-scipy-2022/2584/3) that I'll submit in a few hours, I shared the drafts and asked that the PyGMT authors who are interested in being involved sign on to the abstracts. This is the same approach that @weiji14 took for [EGU 2022](https://forum.generic-mapping-tools.org/t/egu-2022-short-course-proposal/1948). My question is whether it's worth adding recommendations for these types of presentations to [AUTHORSHIP.md](https://github.com/GenericMappingTools/pygmt/blob/main/AUTHORSHIP.md). I think the goals would be to help future developers who haven't presented on software projects before and to make it clear that we would welcome anyone teaching PyGMT without needing to involve all the [PyGMT authors](https://github.com/GenericMappingTools/pygmt/blob/main/AUTHORS.md).