Update README.md in aerosol_concentrations

[szy21]

Checklist:

• [ ] I created a new folder $artifact_name
  • [ ] I added a README.md in that that folder that
  • [ ] describes the data and processing done to it
  • [ ] lists the sources of the raw data
  • [ ] lists the required citation, licenses
  • [ ] If applicable (e.g., for Creative Commons), I added a LICENSE file
  • [ ] I added the scripts that retrieve, process, and produce the artifact
  • [ ] I added the environment used for such scripts (typically, Project.toml and Manifest.toml)
  • [ ] I added the OutputArtifacts.toml file containing the information needed for package developers to add $artifact_name to their package
• [ ] I uploaded the artifact folder to the Caltech cluster (in /groups/esm/ClimaArtifacts/artifacts/$artifact_name)
• [ ] I added the relevant code to the Overides.toml on the Caltech Cluster (in /groups/esm/ClimaArtifacts/artifacts/Overrides.toml)
• [ ] I added a link to the main README.md to point to the new artifact