add dfm readme

polling-config-manager.md

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+
+## Using the SDK
+
+To use the SDK, the Optimizely instance can be initialized in three different ways as per your requirement.
+
+ 1. Initialize Optimizely with a datafile. This datafile will be used as ProjectConfig throughout the life of Optimizely instance.
+    ~~~~~~~~~~~~
+    optimizely.Optimizely(
+      datafile
+    )
+  
+ 2. Initialize Optimizely by providing an 'sdk_key'. This will initialize a PollingConfigManager that makes an HTTP GET request to the URL ( formed using your provided sdk key and the default datafile CDN url template) to asynchronously download the project datafile at regular intervals and update ProjectConfig when a new datafile is recieved. A hard-coded datafile can also be provided along with the sdk_key that will be used initially before any update.
+    ~~~~~~~~~~~~
+    optimizely.Optimizely(
+      datafile=None,
+      sdk_key='put_your_sdk_key_here'
+    )
+
+ 3. Initialize Optimizely by providing a Config Manager that implements a 'get_config' method.You may use our Polling Config Manager and customize it to your need. 
+    ~~~~~~~~~~~~
+    optimizely.Optimizely(
+      config_manager=custom_config_manager
+    )
+
+##### PollingConfigManager
+
+The PollingConfigManager asynchronously polls for datafiles from a specified URL at regular intervals by making HTTP request. 
+
+  polling_config_manager = PollingConfigManager(
+                 sdk_key=None,
+                 datafile=None,
+                 update_interval=None,
+                 url=None,
+                 url_template=None,
+                 logger=None,
+                 error_handler=None,
+                 notification_center=None,
+                 skip_json_validation=False
+    )
+        
+**Note**: One of the sdk_key or url must be provided. When both are provided, url takes the preference.
+
+**sdk_key**
+The sdk_key is used to compose the outbound HTTP request to the default datafile location on the Optimizely CDN.
+
+**datafile**
+You can provide an initial datafile to bootstrap the  `ProjectConfigManager`  so that it can be used immediately. The initial datafile also serves as a fallback datafile if HTTP connection cannot be established. The initial datafile will be discarded after the first successful datafile poll.
+
+**update_interval**
+The update_interval is used to specify a fixed delay in seconds between consecutive HTTP requests for the datafile.
+
+**url_template**
+A string with placeholder `{sdk_key}` can be provided so that this template along with the provided sdk key is used to form the target URL.
+
+You may also provide your own logger, error_handler or notification_center. 
+
+
+###### Advanced configuration
+The following properties can be set to override the default configurations for PollingConfigManager.
+
+| **PropertyName** | **Default Value** | **Description**
+| -- | -- | --
+| update_interval | 5 minutes | Fixed delay between fetches for the datafile 
+| sdk_key | None | Optimizely project SDK key
+| url | None | URL override location used to specify custom HTTP source for the Optimizely datafile. 
+| url_template | 'https://cdn.optimizely.com/datafiles/{sdk_key}.json' | Parameterized datafile URL by SDK key.
+| datafile | None | Initial datafile, typically sourced from a local cached source.
+
+A notification signal will be triggered whenever a _new_ datafile is fetched and Project Config is updated. To subscribe to these notifications you can use the `notification_center.add_notification_listener(NotificationTypes.OPTIMIZELY_CONFIG_UPDATE, update_callback)`
+
+