added missing english text (#1763)

content/altinn-studio/concepts/prefill/_index.en.md

@@ -2,4 +2,23 @@
 title: Prefill
 description: What is prefilling of data?
 weight: 10
----
+---
+
+Prefilling data allows you to provide the end user with a form where relevant fields are already populated with 
+available data. 
+An example of this concept is the Norwegian tax returns. Each year an individuals tax return is prefilled with 
+all the details the Norwegian Tax Administration already have about the individual. The user can then review the 
+information and either confirm or make necessary changes.
+
+<!--Det er viktig å alltid gjøre en vurdering av hvilke data som skal innhentes i et skjema, og om man har hjemmel til
+å hente inn disse dataene. All data som hentes inn skal ha et formål.
+> TODO: Få en jurist til å skrive noen linjer om hva som gjelder her mtp forhåndsutfylling.-->
+
+
+In Altinn Studio there are several ways to prefill a form.
+- To show data from the National Population Register (DSF), the Central Coordinating Register (ER) or the users profile
+  in Altinn, you can set up prefill via static configuration files.
+- Data that is available via external API's can be fetched during startup of a new instance of an app, and added
+  to the form via backend code.
+- An app can be started directly via API's, and it is then possible to include (complete or partial) form data as part
+  of the request.