Document SMS class.

config/models.py

@@ -335,6 +335,16 @@ class Screen(Base, AWAREsensor):
 
 
 class SMS(Base, AWAREsensor):
+    """
+    Contains the messages sensor information.
+
+    Attributes
+    ----------
+    message_type: int
+        Message type (1 – received, 2 – sent)
+    trace: str(40)
+        A hash value SHA-1 of the phone number (source or target) of the call
+    """
     message_type = Column(SmallInteger, nullable=False)
     trace = Column(String(length=40), nullable=False)