Link to try-gmt binder repository

[weiji14]

Description of proposed changes

Allow potential users to try PyGMT directly without installing anything on their computer, just with two clicks!

Fixes #

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.

[seisman]

OK to me, but note that it won't be shown in the PyGMT website.

[weiji14]

Yep, let's leave it to the next release. The front page does need a good redesign.