Peer Discovery Interface (#123)

* added file

* basic interface modeled on go repo

* fixed linting

* updated based on comments

libp2p/discovery/advertiser_interface.py

@@ -0,0 +1,16 @@
+from abc import ABC, abstractmethod
+# pylint: disable=too-few-public-methods
+
+class IAdvertiser(ABC):
+
+    def __init__(self):
+        pass
+
+    @abstractmethod
+    def advertise(self, service):
+        """
+        Advertise providing a specific service to the network
+        :param service: service that you provide
+        :raise Exception: network error
+        """
+