Document generic data sources feature

Explain how to enable the any data source feature and briefly
outline data population.

content/en/docs/concepts/storage/persistent-volumes.md

@@ -710,6 +710,27 @@ spec:
       storage: 10Gi
 ```
 
+## Data Population
+
+{{< feature-state for_k8s_version="v1.18" state="alpha" >}}
+
+Data population is an experimental feature to allow persistent volumes to be created non-empty,
+from sources other than volumes or snapshots.
+
+To enable support for data population,  enable the `AnyVolumeDataSource` feature gate on the
+apiserver.
+
+This feature allows specifying any custom resource as the `DataSource` of a new PVC, as long
+as the custom resource is in the same namespace, and there is a controller installed to handle
+data population from that custom resource type (CRD). It's important to note that ordinary
+dynamic provisioners will ignore PVCs with `DataSource` types they don't support, so users
+need to be aware of what custom resources can be used for data population, and what
+controllers are installed.
+
+Users may create their own data populator CRDs and write their own data populator controllers
+to handle them. Such data populators are responsible for creating the persistent volume and
+binding it to the PVC on their own.
+
 ## Writing Portable Configuration
 
 If you're writing configuration templates or examples that run on a wide range of clusters