Merge pull request #265 from andypbarrett/add-download-onprem-howto

add download from onprem how-to to documentation

docs/tutorials/onprem.md

@@ -1,3 +1,23 @@
 # Download data for on-prem datasets 
 
-Coming soon
+Import the `earthaccess` library, search for granules in an on-prem dataset and download to a local computer.
+The Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data, Version 2, dataset is used.
+This has the shortname NSIDC-0051.  We search for and download data granules for the northern hemisphere for November 2023.
+In this example, granules are downloaded to the current working directory using the `local_path` keyword.  This can be changed to your preferred download location.
+
+```py
+import earthaccess
+
+auth = earthaccess.login()
+
+results = earthaccess.search_data(
+    short_name='NSIDC-0051',
+    temporal=('2022-11-01', '2022-11-30'),
+    bounding_box=(-180, 0, 180, 90)
+)
+
+downloaded_files = earthaccess.download(
+    results,
+    local_path='.',
+)
+```