updated with output binding support (#652)

reference/specs/bindings/twitter.md

@@ -1,5 +1,7 @@
 # Twitter Binding Spec
 
+The Twitter binding supports both `input` and `output` binding configuration. First the common part:
+
 ```yaml
 apiVersion: dapr.io/v1alpha1
 kind: Component
@@ -17,6 +19,42 @@ spec:
     value: "****" # twitter api access token, required
   - name: accessSecret
     value: "****" # twitter api access secret, required
+```
+
+For input bindings, where the query matching Tweets are streamed to the user service, the above component has to also include a query: 
+
+```yaml
   - name: query
     value: "dapr" # your search query, required 
 ```
+
+For output binding invocation the user code has to invoke the binding:
+
+```shell
+POST http://localhost:3500/v1.0/bindings/twitter
+```
+
+Where the payload is:
+
+```json
+{
+  "data": "",
+  "metadata": {
+    "query": "twitter-query",
+    "lang": "optional-language-code",
+    "result": "valid-result-type"
+  },
+  "operation": "get"
+}
+```
+
+The metadata parameters are:
+
+* `query` - any valid Twitter query (e.g. `dapr` or `dapr AND serverless`). See [Twitter docs](https://developer.twitter.com/en/docs/tweets/rules-and-filtering/overview/standard-operators) for more details on advanced query formats 
+* `lang` - (optional, default: `en`) restricts result tweets to the given language using [ISO 639-1 language code](https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes)
+* `result` - (optional, default: `recent`) specifies tweet query result type. Valid values include:
+  * `mixed` - both popular and real time results
+  * `recent` - most recent results
+  * `popular` - most popular results
+
+You can see the example of the JSON data that Twitter binding returns [here](https://developer.twitter.com/en/docs/tweets/search/api-reference/get-search-tweets)