Add files listing package maintainers

[santisoler]

Description:

After being discussed in the Fatiando Call of 2024-01-18, we noticed that we don't have a public list of the maintainers for each package.

To tackle this, we decided to add a MAINTAINERS.md file on each repo listing them. Its format could be similar to the ones for AUTHORS.md, although I don't think we need to add affiliation or DOI.

Something like this could work:

# Project maintainers

- [First Last Name](https://github.com/github-handle)

Apply to:

• boule
• harmonica
• pooch
• verde
• ensaio
• choclo
• dependente

Further instructions:

• Start by opening Pull Requests on each repository listed above.
• Optionally, we can open Issues on each repository if further discussion specific to that repository is needed.
• Mention this Issue on every Issue or Pull Request opened on each opened: Related to fatiando/community#137
• Check-off the repository on the list above once the Pull Request is merged.
• Close this issue when all items are checked-off.

We want your help!

We know that maintenance tasks are very demanding, so we don't expect a single person to tackle this issue by themselves. Any help is very welcomed, so please comment below that you want to take care of the changes on any repository and we will assign it to you.

[leouieda]

I'm wondering if this is worth doing or if we should list them in the README instead. It won't be a long list and it's more visible in the README. Same goes for the authors files now that I think about it.