Add Liam's ROSES 2020 PyGMT talk to website

[weiji14]

Description of proposed changes

Update main README and Overview page to include the PyGMT talk @liamtoney gave at Remote Online Sessions for Emerging Seismologists (ROSES) 2020. Preview at https://pygmt-git-roses2020talk.gmt.vercel.app

I've also some extra text to the main README, following the example at https://github.com/MarcSkovMadsen/awesome-panel/blob/master/README.md.

Links:

• https://www.youtube.com/watch?v=SSIGJEe0BIk
• https://www.iris.edu/hq/inclass/lesson/728
• https://github.com/fdannemanndugick/roses2020/tree/master/unit8

References on how youtube thumbnails work:

• https://stackoverflow.com/questions/11804820/how-can-i-embed-a-youtube-video-on-github-wiki-pages
• https://stackoverflow.com/questions/2068344/how-do-i-get-a-youtube-video-thumbnail-from-the-youtube-api

Closes #613

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]

Looks great!