Update the README.rst with circular queue mode info and disclaimer.

Author: panta

README.rst

@@ -348,6 +348,18 @@ or for the python logging interface:
 sure the communication thread terminates and it's joined correctly. Otherwise the program won't exit, waiting for
 the thread, unless forcibly killed.
 
+#### Circular queue mode
+
+In some applications it can be especially important to guarantee that the logging process won't block under *any*
+circumstance, even when it's logging faster than the sending thread could handle (_backpressure_). In this case it's
+possible to enable the `circular queue` mode, by passing `True` in the `queue_circular` parameter of
+``asyncsender.FluentSender``. By doing so the thread doing the logging won't block even when the queue is full, the
+new event will be added to the queue by discarding the oldest one.
+
+**WARNING**: setting `queue_circular` to `True` will cause loss of events if the queue fills up completely! Make sure
+that this doesn't happen, or it's acceptable for your application.
+
+
 Testing
 -------