Document broker compatibility (as suggested in #24)

Author: edenhill

README.md

@@ -48,6 +48,29 @@ c.close()
 See [examples](examples) for more examples.
 
 
+Broker compatibility
+====================
+The Python client (as well as the underlying C library librdkafka) supports
+all broker versions >= 0.8.
+But due to the nature of the Kafka protocol in broker versions 0.8 and 0.9 it
+is not safe for a client to assume what protocol version is actually supported
+by the broker, thus you will need to hint the Python client what protocol
+version it may use. This is done through two configuration settings:
+
+ * `broker.version.fallback=YOUR_BROKER_VERSION` (default 0.9.0.1)
+ * `api.version.request=true|false` (default false)
+
+When using a Kafka 0.10 broker or later you only need to set
+`api.version.request=true`.
+If you use Kafka broker 0.9 or 0.8 you should leave
+`api.version.request=false` (default) and set
+`broker.version.fallback` to your broker version,
+e.g `broker.version.fallback=0.9.0.1`.
+
+More info here:
+https://github.com/edenhill/librdkafka/wiki/Broker-version-compatibility
+
+
 Prerequisites
 =============