Update README.md with basic usage example

Author: mhidas

README.md

@@ -1,2 +1,63 @@
 # aodn-netcdf-tools
 Repository for templates and code relating to generating standard NetCDF files for the Australia Ocean Data Network
+
+## ncwrwiter
+
+This module provides a basic templating tool for creating netCDF files based on templates.
+
+Templates can be provided as individual Python dictionaries, or as a JSON file, e.g. `template.json`:
+```json
+{
+    "dimensions": {
+        "TIME": 0
+    },
+    "variables": {
+        "TIME": {
+            "dimensions": ["TIME"],
+            "type": "float64"
+        }
+    },
+    "global_attributes": {
+        "title": "test dataset"
+    }
+}
+```
+Basic usage
+```python
+from ncwriter import DatasetTemplate
+
+# create template from individual dictionaries
+template = DatasetTemplate(dimensions=dims_dict, variables=var_dict, global_attributes=gatt_dict)
+# OR from a template file
+template = DatasetTemplate.from_json('template.json')
+
+# add/update attributes
+template.global_attributes.update({
+    'comment': 'this was added later',
+    'date_created': '2018-09-20T00:00:00Z'
+    })
+
+# add/update variables
+template.variables["TIME"]["atttributes"] = {
+    "standard_name": "time",
+    "units": "days since 1950-01-01T00:00:00Z"
+}
+template.variables["TEMP"] = {
+    "dimensions": ["TIME"],
+    "type": "float32",
+    "attributes": {
+        "standard_name": "sea_water_temperature",
+        "units": "degC",
+        "valid_min": 0.0,
+        "valid_max": 42.00
+    }
+}
+
+# add variable values (automatically updates size of corresponding dimensions)
+template.variables['TIME']['values'] = np.arange(10)
+template.variables['TEMP']['values'] = np.array([12.1, 12.2, 12.3, 12.4, 12.5, 12.6, 12.7, 12.8, 12.9, 13.0])
+
+# create netCDF file
+template.to_netcdf('example.nc')
+
+```