add Geodynamics Underworld + mattermost

[leomiquelutti]

Project name: Underworld

Project website/repository: https://github.com/underworldcode/underworld2

License: mixed (LGPL-3 + C.C> 4.0)

Submission type:

New Software Project + Fix/update existing entry

Checklist for all pull requests:

• [x ] Project is certified awesome

• [x ] The project is open-source and accessible.
• [x ] Searched the existing entries to make sure this is not a duplicate.
• [x ] Contains only a single addition (make separate PRs if adding more than one).
• [x ] Added the link to the bottom of the relevant category.
• [x ] Created a new category only if necessary.

• Add icon from the media/icon/ folder if applicable (e.g., ![Python](media/icon/python.png) ).
• Checked spelling and grammar.
• Removed trailing whitespace and periods.
• Confirm the dash – is not a minus -.
• Used title-casing (AP style) for project name.

Checklist for new software projects:

• [x ] Link the code repository rather than the website/documentation
• [x ] Installation instructions present
• [x ] Documentation looks great
• Examples or Tutorials to follow
• Tests and Travis CI running

Checklist for new Data Repositories, Tutorials and Cheat Sheets, and Miscellaneous:

• Easily accessible
• Comprehensive and widely appealing

[JustinGOSSES]

Looks good