Add line to FAQ about blocking in a callback (#497)

Author: sbSteveK

documents/FAQ.md

@@ -32,6 +32,9 @@ This could be many different things but it most likely is a policy issue. Start
 
 After getting it working make sure to only allow the actions and resources that you need. More info about IoT IAM policies can be found [here](https://docs.aws.amazon.com/iot/latest/developerguide/security_iam_service-with-iam.html).
 
+### I am experiencing deadlocks
+You MUST NOT perform blocking operations on any callback, or you will cause a deadlock. For example: in the on_publish_received callback, do not send a publish, and then wait for the future to complete within the callback. The Client cannot do work until your callback returns, so the thread will be stuck.
+
 ### Mac-Only TLS Behavior
 
 Please note that on Mac, once a private key is used with a certificate, that certificate-key pair is imported into the Mac Keychain.  All subsequent uses of that certificate will use the stored private key and ignore anything passed in programmatically.  Beginning in v1.7.3, when a stored private key from the Keychain is used, the following will be logged at the "info" log level: