adapted README.md to include soilgrids_ocd

Author: EvaMarieMetz

README.md

@@ -50,6 +50,7 @@ artifacts.
 - [Spun up initial conditions for soil, snow, canopy, with no flux at the bottom](https://github.com/CliMA/ClimaArtifacts/tree/main/saturated_land_ic)
 - [CLM surface data for the year 2000](https://github.com/CliMA/ClimaArtifacts/tree/main/clm_data)
 - [Soil texture parameters needed for the full soil model; derived from SoilGrids](https://github.com/CliMA/ClimaArtifacts/tree/main/soilgrids)
+- [Soil crganic carbon densities for the soil co2 model; derived from SoilGrids](https://github.com/CliMA/ClimaArtifacts/tree/main/soilgrids_ocd)
 - [Soil parameters needed for Richards equation; global at 1km resolution from
 S. Gupta et al 2022, 2024](https://github.com/CliMA/ClimaArtifacts/tree/main/soil_params_Gupta2020_2022)
 - [Foliage clumping index, derived from MODIS data for 2006](https:////github.com/CliMA/ClimaArtifacts/tree/main/modis_clumping_index)