added help message

Author: yankev

plotly/session.py

@@ -76,9 +76,11 @@ def sign_in(username, api_key, **kwargs):
     # raise error if key isn't valid anywhere
     for key in kwargs:
         if key not in CREDENTIALS_KEYS and key not in CONFIG_KEYS:
-            raise exceptions.PlotlyError(
-                "{} is not a valid config or credentials key".format(key)
-            )
+            raise exceptions.PlotlyError("{} is not a valid config or "
+                                         "credentials key\n"
+                                         "Type help(sign_in) for a full list "
+                                         "of possbile sign in related "
+                                         "keys".format(key))
 
     # add credentials, raise error if type is wrong.
     for key in CREDENTIALS_KEYS: