Add "Who is using Fatiando?" section

[santisoler]

Add new section listing software projects that use some Fatiando
libraries.

Working on #11

Reminders:

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
• Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.

[leouieda]

@santisoler I'm wondering if this is a list that's worth maintaining, since it can change quite a bit. I don't expect most of our software will widely used by other software (probably only pooch and boule). I see the front page as a advertisement for end users, who are probably more interested in uses in papers and teaching.

I'm not opposed to adding this, by the way. Just raising the point that the list will have to be maintained by us.

[santisoler]

You are right, as I pointed out on #11 my idea was to add publications on the website.
So far I started adding those libraries that are using Pooch, but probably this is too specific and much more developer-oriented, rather than targeted to end users.

We can leave this here for now and discuss about it later, maybe on a Community Call.
For the record, I'm not entirely sure if we should add this or not, but I like how opening PRs encourage these discussions.

[leouieda]

@santisoler closing this as it may be hard to update to master. Probably easier to redo it. Feel free to reopen if you prefer.