Re-add part3

advanced/part3/src/Log.elm

@@ -0,0 +1,20 @@
+module Log exposing (error)
+
+{-| This is a placeholder API for how we might do logging through
+some service like <http://rollbar.com> (which is what we use at work).
+
+Whenever you see Log.error used in this code base, it means
+"Something unexpected happened. This is where we would log an
+error to our server with some diagnostic info so we could investigate
+what happened later."
+
+(Since this is outside the scope of the RealWorld spec, and is only
+a placeholder anyway, I didn't bother making this function accept actual
+diagnostic info, authentication tokens, etc.)
+
+-}
+
+
+error : Cmd msg
+error =
+    Cmd.none