Add an AGENTS.md file

As AI coding agents such as GitHub Copilot, Claude Code, Codex, and Cursor become more widely used, it may be helpful for PyGMT to provide an AGENTS.md file with repository-specific instructions.

The file could briefly describe:

- The purpose and structure of PyGMT
- How to set up a development environment
- Testing requirements
- Coding and documentation conventions
- Useful links to contributor and developer documentation
- More

Many projects are starting to adopt AGENTS.md files to help AI agents understand project workflows and produce contributions that better follow project standards.

References:

- https://agents.md/