Update Seismic tools section in README.md with Synthoseis package

[mmcint]

adding synthoseis package to seismic section.

this package, which was created and open-sourced by Shell, generates synthetic seismic data based off a given rock properties model.

Project name: Synthoseis

Project website/repository: https://github.com/sede-open/synthoseis

License: MIT License

Submission type: New Software Project

Checklist for all pull requests:

• Project is certified awesome

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

• Link the code repository rather than the website/documentation
• Installation instructions present
• 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

[mmcint]

@JustinGOSSES looks like this is ready for your approval

[JustinGOSSES]

Cool project