Added Event Grid binding spec (#569)

reference/specs/bindings/eventgrid.md

@@ -0,0 +1,67 @@
+# Azure Event Grid Binding Spec
+
+See [this](https://docs.microsoft.com/en-us/azure/event-grid/) for Azure Event Grid documentation.
+
+```yml
+apiVersion: dapr.io/v1alpha1
+kind: Component
+metadata:
+  name: <name>
+spec:
+  type: bindings.azure.eventgrid
+  metadata:
+  # Required Input Binding Metadata
+  - name: tenantId
+    value: "[AzureTenantId]"
+  - name: subscriptionId
+    value: "[AzureSubscriptionId]"
+  - name: clientId
+    value: "[ClientId]"
+  - name: clientSecret
+    value: "[ClientSecret]"
+  - name: subscriberEndpoint
+    value: "[SubscriberEndpoint]"    
+  - name: handshakePort
+    value: [HandshakePort]
+  - name: scope
+    value: "[Scope]"
+
+  # Optional Input Binding Metadata
+  - name: eventSubscriptionName
+    value: "[EventSubscriptionName]"
+
+  # Required Output Binding Metadata
+  - name: accessKey
+    value: "[AccessKey]"
+  - name: topicEndpoint
+    value: "[TopicEndpoint]
+```
+
+## Input Binding Metadata
+- `tenantId` is the Azure tenant id in which this Event Grid Event Subscription should be created
+
+- `subscriptionId` is the Azure subscription id in which this Event Grid Event Subscription should be created
+
+- `clientId` is the client id that should be used by the binding to create or update the Event Grid Event Subscription
+
+- `clientSecret`  is the client secret that should be used by the binding to create or update the Event Grid Event Subscription
+
+- `subscriberEndpoint` is the https (required) endpoint in which Event Grid will handshake and send Cloud Events. If you aren't re-writing URLs on ingress, it should be in the form of: `https://[YOUR HOSTNAME]/api/events` If testing on your local machine, you can use something like [ngrok](https://ngrok.com) to create a public endpoint. 
+
+- `handshakePort` is the container port that the input binding will listen on for handshakes and events
+
+- `scope` is the identifier of the resource to which the event subscription needs to be created or updated. The scope can be a subscription, or a resource group, or a top level resource belonging to a resource provider namespace, or an Event Grid topic. For example:
+    - '/subscriptions/{subscriptionId}/' for a subscription
+    - '/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}' for a resource group
+    - '/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/{resourceProviderNamespace}/{resourceType}/{resourceName}' for a resource
+    - '/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.EventGrid/topics/{topicName}' for an Event Grid topic
+    > Values in braces {} should be replaced with actual values.
+
+- `eventSubscriptionName` (Optional) is the name of the event subscription. Event subscription names must be between 3 and 64 characters in length and should use alphanumeric letters only.
+
+## Output Binding Metadata
+- `accessKey` is the Access Key to be used for publishing an Event Grid Event to a custom topic
+
+- `topicEndpoint` is the topic endpoint in which this output binding should publish events
+
+> **Note:** In production never place passwords or secrets within Dapr components. For information on securely storing and retrieving secrets refer to [Setup Secret Store](../../../howto/setup-secret-store)