Add underscore guideline to CONTRIBUTING.md

[willschlitzer]

As mentioned by @core-man in #1024, this adds to the convention of underscores between words in file names in the contributing guidelines.

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.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[seisman]

I think we should apply the underscore rule to directories. For example, a URL like the one below is pretty weird.

https://www.pygmt.org/dev/gallery/3d-plots/3d_perspective_view.html

[willschlitzer]

I think we should apply the underscore rule to directories. For example, a URL like the one below is pretty weird.

https://www.pygmt.org/dev/gallery/3d-plots/3d_perspective_view.html

Added in e9102b3