Expand README with details of storage use

Add a note about defining `method` paramter for the datastore_upsert endpoint.

README.md

@@ -25,6 +25,8 @@ pip install tableschema-ckan-datastore
 
 Code examples in this readme requires Python 3.3+ interpreter. You could see even more example in [examples](https://github.com/frictionlessdata/tableschema-ckan-datastore-py/tree/master/examples) directory.
 
+When writing data, tableschema-ckan-datastore uses the [CKAN API `datastore_upsert` endpoint](https://ckan.readthedocs.io/en/latest/maintaining/datastore.html#ckanext.datastore.logic.action.datastore_upsert) with the `upsert` method. This requires a unique key in the data defined by a [Table Schema primary key property](https://specs.frictionlessdata.io/table-schema/#primary-key). If your data has a primary key, you can use the `table.save` method:
+
 ```python
 from tableschema import Table
 
@@ -34,14 +36,39 @@ base_url = 'https://demo.ckan.org'
 dataset_id = 'test-dataset-010203'
 api_key = 'my-ckan-user-api-key'
 
-table = Table('data.csv', schema='schema.json')
+table = Table('data.csv', schema='schema.json')  # data.csv has primary keys
 table.save(resource_id,
            storage='ckan_datastore',
            base_url=base_url,
            dataset_id=dataset_id,
            api_key=api_key)
 ```
 
+If you need to define the method used to save data to the DataStore, you can create the `tableschema.Storage` object directly and specify which method parameter to use:
+
+```python
+import io
+import json
+from tabulator import Stream
+from tableschema import Storage
+
+# Load and save CKAN DataStore record
+resource_id = 'bd79c992-40f0-454a-a0ff-887f84a792fb'
+base_url = 'https://demo.ckan.org'
+dataset_id = 'test-dataset-010203'
+api_key = 'my-ckan-user-api-key'
+
+schema = json.load(io.open('schema.json', encoding='utf-8'))
+data = Stream('data.csv', headers=1).open()
+
+storage = Storage.connect('ckan_datastore',
+                          base_url=base_url,
+                          dataset_id=dataset_id,
+                          api_key=api_key)
+storage.create(resource_id, schema, force=True))
+storage.write(resource_id, data, method='insert')  # specify the datastore_upsert method
+```
+
 ## Documentation
 
 The whole public API of this package is described here and follows semantic versioning rules. Everything outside of this readme are private API and could be changed without any notification on any new version.
@@ -102,7 +129,7 @@ and `mock` packages. This packages are available only in tox envionments.
 
 ## Changelog
 
-Here described only breaking and the most important changes. The full changelog and documentation for all released versions could be found in nicely formatted [commit history](https://github.com/frictionlessdata/tableschema-sql-py/commits/master).
+Here described only breaking and the most important changes. The full changelog and documentation for all released versions could be found in nicely formatted [commit history](https://github.com/frictionlessdata/tableschema-ckan-datastore-py/commits/master).
 
 ### v0.x