Merge pull request grpc#22844 from muxi/cfstream-doc

Update CFStream doc with CFRunLoop poller info

Author: muxi

src/objective-c/README-CFSTREAM.md

@@ -23,8 +23,18 @@ platforms, you can turn it on with macro `GRPC_CFSTREAM=1` for the pod 'gRPC-Cor
 case of problem and you want to disable CFStream on iOS, you can set environment variable
 "grpc\_cfstream=0".
 
+## Caveats
+It is known to us that the CFStream API has some bug (FB6162039) which will cause gRPC's CFStream
+networking layer to stall occasionally. The issue mostly occur on MacOS systems (including iOS
+simulators on MacOS); iOS may be affected too but we have not seen issue there. gRPC provides a
+workaround to this problem with an alternative poller based on CFRunLoop. The poller can be enabled
+by setting environment variable `GRPC_CFSTREAM_RUN_LOOP=1`. Note that the poller is a client side
+only poller that does not support running a server on it. That means if an app opts in to the
+CFRunLoop-based poller, the app cannot host a gRPC server (gRPC Objective-C does not support running
+a server but other languages running on iOS do support it).
+
 ## Notes
 
 - Currently we do not support platforms other than iOS, although it is likely that this integration
   can run on MacOS targets with Apple's compiler.
-- Let us know if you meet any issue by filing issue and ping @muxi.
+- Let us know if you meet any issue by filing issue and ping @stanleycheung.