Update README.md

[abuseki]

Add EarthChem

Project name: EarthChem

Project website/repository: https://www.earthchem.org/

License: Five CC licenses are allowed for contributons: CC BY-SA 4.0 recommended and default for submissions of new data sets.\ Other CC licenses contributors can choose from: CC BY 4.0, CC BY-NC-SA 4.0, CC BY-NC-SA 3.0 US, CC0.

Submission type: New Data Repository

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.

• [X] Add icon from the media/icon/ folder if applicable (e.g., ![Python](media/icon/python.png) ).
• [X] Checked spelling and grammar.
• [X] Removed trailing whitespace and periods.
• [X] Confirm the dash – is not a minus -.
• [X] 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:

• [X] Easily accessible
• [X] Comprehensive and widely appealing

[mmcint]

@abuseki I see that you're adding several links here. are these all associated with earthchem?

@JustinGOSSES we typically try to keep a single project/repo to one commit, correct?

[JustinGOSSES]

Typically one contribution at a time is best. This is because one contribution might not qualify or take longer.

However, probably not worth breaking up this PR once already submitted unless we see a problem with any of the contributions.

I can take the geochron ones as I used the excel addon back in the day and would like to check them out.

[JustinGOSSES]

Fixed two URL bugs with issues unrelated to this pull request but that had to be fixed to get CI to pass.

[JustinGOSSES]

Thanks @abuseki and @mmcint