Add steps to trigger a manual transform to README

Document how to perform a manual transform, which is useful if the
output data set is missing data.

README.md

@@ -83,6 +83,38 @@ command
 curl -X PUT -d "[]" https://{backdrop_url}/data/<data-group>/<data-type> -H 'Authorization: Bearer <token-from-stagecraft>' -H 'Content-Type: application/json'
 ```
 
+## Triggering a transform manually
+
+A transform occurs when data is written to in Backdrop. The transform applies
+calculations to the data and writes the results to a second dataset.
+
+You may wish to trigger a transform manually if data is missing from a output
+data set.
+
+Transformations are configured in [Stagecraft][] via the API or Django admin
+application.
+
+[Stagecraft]: https://github.com/alphagov/stagecraft
+
+1. Log in to the Stagecraft Django admin application to obtain a bearer token
+   for the source data set:
+
+    a. Select 'Data sets' from the 'Datasets' section in the main menu.
+    b. Search for the source data set
+    c. Make a note of the data group and data type for the data set you wish to
+      transform
+    d. Click on the name of the data set
+    e. Copy the bearer token from the form field
+
+1. Run the following command, replacing the fields in capitals:
+
+   ```
+   curl-H 'Authorization: Bearer <INSERT BEARER TOKEN HERE>' \
+     -H 'content-type: application/json' \
+     -d '{"_start_at": "2012-01-01T00:00:00Z", "_end_at": "2015-03-20T00:00:00Z"}' \
+     https://www.performance.service.gov.uk/data/<DATA GROUP>/<DATA TYPE>/transform
+   ```
+
 ## Celery worker
 
 Backdrop uses celery for running tasks on data post write - these can be found