Add PyGMT AGU 2019 poster to website

[weiji14]

Description of the desired feature

The presentations on our website are slightly out of date, and we'll need someone to help add the 2019 AGU Poster to the page!

We'll need edit the overview page to add the details of the new poster including:

• Poster title
• Authors
• Presented at 'Conference'
• DOI link

Next is to upload a static figure of the poster, You can find the JPG file at https://github.com/leouieda/website/blob/master/images/poster-agu2019.jpg.

You can follow the format of the old examples, and be sure to give a shout out if you need help! Find out more about contributing in our CONTRIBUTING.md docs. Once you create a pull request, we'll be able to preview any changes you made and guide you along 😄

Are you willing to help implement and maintain this feature? Yes/No

[leouieda]

Thanks for this, @weiji14!