add example

README.md

@@ -1,3 +1,54 @@
 # ZarrDatasets
 
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+
+
+ZarrDatasets.jl is a package to read Zarr datasets using the native julia implementation [Zarr.jl](https://github.com/JuliaIO/Zarr.jl)
+using the [CommonDataModel.jl](https://github.com/JuliaGeo/CommonDataModel.jl) interface.
+
+In the following example data from [Copernicus marine service](https://marine.copernicus.eu/) are loaded using `ZarrDatasets` and a subset
+is saved as a NetCDF file:
+
+
+```julia
+using CommonDataModel: @select
+using Dates
+using NCDatasets
+using STAC
+using ZarrDatasets
+
+# get the data set URL from product_id and dataset_id and the STAC catalog
+function copernicus_marine_catalog(product_id,dataset_id,
+    stac_url = "https://stac.marine.copernicus.eu/metadata/catalog.stac.json",
+    asset = "timeChunked")
+
+    cat  = STAC.Catalog(stac_url);
+    item_canditates = filter(startswith(dataset_id),keys(cat[product_id].items))
+    # use last version per default
+    dataset_version_id = sort(item_canditates)[end]
+    item = cat[product_id].items[dataset_version_id]
+    return href(item.assets[asset])
+end
+
+product_id = "MEDSEA_MULTIYEAR_PHY_006_004"
+dataset_id = "med-cmcc-ssh-rean-d"
+
+url = copernicus_marine_catalog(product_id,dataset_id)
+# surprisingly requesting missing data chunks results in the HTTP error
+# code 403 (permission denied) rather than 404 (not found) for the CMEMS server.
+ds = ZarrDataset(url,_omitcode=[404,403]);
+
+# longitude, latitude and time are the coordinate variables defined in the
+# zarr dataset
+ds_sub = @select(ds, time == DateTime(2001,1,1)
+    && 7 <= longitude <= 11
+    && 42.3 <= latitude <= 44.5)
+
+# save selection as NetCDF file
+NCDataset("$(dataset_id)_selection.nc","c") do ds_nc
+    write(ds_nc,ds_sub)
+end
+```