Add documentation for Preview Features (#1464)

* Add documentation for Preview Features

Add an introduction to preview features with examples of how
to set/specify them in the dapr configuration.

https://github.com/dapr/docs/issues/1463

* Update daprdocs/content/en/operations/configuration/configuration-overview.md

Co-authored-by: Aaron Crawfis <Aaron.Crawfis@microsoft.com>

* Update daprdocs/content/en/operations/configuration/preview-features.md

Co-authored-by: Aaron Crawfis <Aaron.Crawfis@microsoft.com>

* Update daprdocs/content/en/operations/configuration/preview-features.md

Co-authored-by: Aaron Crawfis <Aaron.Crawfis@microsoft.com>

* Update daprdocs/content/en/operations/configuration/preview-features.md

Co-authored-by: Aaron Crawfis <Aaron.Crawfis@microsoft.com>

* Update daprdocs/content/en/operations/configuration/preview-features.md

Co-authored-by: Aaron Crawfis <Aaron.Crawfis@microsoft.com>

* Update daprdocs/content/en/operations/configuration/preview-features.md

Co-authored-by: Aaron Crawfis <Aaron.Crawfis@microsoft.com>

* Update daprdocs/content/en/operations/configuration/preview-features.md

Co-authored-by: Aaron Crawfis <Aaron.Crawfis@microsoft.com>

* Update daprdocs/content/en/operations/configuration/preview-features.md

Co-authored-by: Aaron Crawfis <Aaron.Crawfis@microsoft.com>

* Update daprdocs/content/en/operations/configuration/preview-features.md

Co-authored-by: Aaron Crawfis <Aaron.Crawfis@microsoft.com>

* Update daprdocs/content/en/operations/configuration/preview-features.md

Co-authored-by: Aaron Crawfis <Aaron.Crawfis@microsoft.com>

* Update daprdocs/content/en/operations/configuration/preview-features.md

Co-authored-by: Aaron Crawfis <Aaron.Crawfis@microsoft.com>

* Update daprdocs/content/en/operations/configuration/preview-features.md

Co-authored-by: Aaron Crawfis <Aaron.Crawfis@microsoft.com>

Co-authored-by: Aaron Crawfis <Aaron.Crawfis@microsoft.com>

daprdocs/content/en/operations/configuration/configuration-overview.md

@@ -125,6 +125,9 @@ See the [Scoping secrets]({{< ref "secret-scope.md" >}}) guide for information a
 
 See the [Allow lists for service invocation]({{< ref "invoke-allowlist.md" >}}) guide for information and examples on how to set allow lists.
 
+#### Preview features
+See the [preview features]({{< ref "preview-features.md" >}}) guide for information and examples on how to use opt-in preview features.
+
 ### Example sidecar configuration
 The following yaml shows an example configuration file that can be applied to an applications' Dapr sidecar.
 

daprdocs/content/en/operations/configuration/preview-features.md

@@ -0,0 +1,97 @@
+---
+type: docs
+title: "How-To: Enable preview features"
+linkTitle: "Preview features"
+weight: 7000
+description: "How to specify and enable preview features"
+---
+
+## Overview
+Some features in Dapr are considered experimental when they are first released. These features require explicit opt-in in order to be used. The opt-in is specified in Dapr's configuration.
+
+Currently, preview features are enabled on a per application basis when running on Kubernetes. A global scope may be introduced in the future should there be a use case for it.
+
+### Current preview features
+Below is a list of existing preview features:
+- Actor Reentrancy
+
+## Configuration properties
+The `features` section under the `Configuration` spec contains the following properties:
+
+| Property       | Type   | Description |
+|----------------|--------|-------------|
+|name|string|The name of the preview feature that will be enabled/disabled
+|enabled|bool|Boolean specifying if the feature is enabled or disabled
+
+## Enabling a preview feature
+Preview features are specified in the configuration. Here is an example of a full configuration that contains multiple features:
+
+```yaml
+apiVersion: dapr.io/v1alpha1
+kind: Configuration
+metadata:
+  name: featureconfig
+spec:
+  tracing:
+    samplingRate: "1"
+    zipkin:
+      endpointAddress: "http://zipkin.default.svc.cluster.local:9411/api/v2/spans"
+  features:
+    - name: Feature1
+      enabled: true
+    - name: Feature2
+      enabled: true
+```
+
+### Standalone
+To enable preview features when running Dapr locally, either update the default configuration or specify a separate config file using `dapr run`.
+
+The default Dapr config is created when you run `dapr init`, and is located at:
+- Windows: `%USERPROFILE%\.dapr\config.yaml`
+- Linux/macOS: `~/.dapr/config.yaml`
+
+Alternately, you can update preview features on all apps run locally by specifying the `--config` flag in `dapr run` and pointing to a separate Dapr config file:
+
+```bash
+dapr run --app-id myApp --config ./previewConfig.yaml ./app
+```
+
+
+### Kubernetes
+In Kubernetes mode, the configuration must be provided via a configuration component. Using the same configuration as above, apply it via `kubectl`:
+
+```bash
+kubectl apply -f previewConfig.yaml
+```
+
+This configuration component can then be referenced in any application by modifying the application's configuration to reference that specific configuration component via the `dapr.io/config` element. For example:
+
+```yaml
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: nodeapp
+  labels:
+    app: node
+spec:
+  replicas: 1
+  selector:
+    matchLabels:
+      app: node
+  template:
+    metadata:
+      labels:
+        app: node
+      annotations:
+        dapr.io/enabled: "true"
+        dapr.io/app-id: "nodeapp"
+        dapr.io/app-port: "3000"
+        dapr.io/config: "featureconfig"
+    spec:
+      containers:
+      - name: node
+        image: dapriosamples/hello-k8s-node:latest
+        ports:
+        - containerPort: 3000
+        imagePullPolicy: Always
+```