[exporter/kafka] Update README.md (#19650)

Update README.md

Updated documentation to reflect the changes made in (this PR)[#5763]

exporter/kafkaexporter/README.md

@@ -31,7 +31,9 @@ The following settings can be optionally configured:
   - `sasl`
     - `username`: The username to use.
     - `password`: The password to use
-    - `mechanism`: The sasl mechanism to use (SCRAM-SHA-256, SCRAM-SHA-512 or PLAIN)
+    - `mechanism`: The sasl mechanism to use (SCRAM-SHA-256, SCRAM-SHA-512, AWS_MSK_IAM or PLAIN)
+    - `aws_msk.region`: AWS Region in case of AWS_MSK_IAM mechanism
+    - `aws_msk.broker_addr`: MSK Broker address in case of AWS_MSK_IAM mechanism
   - `tls`
     - `ca_file`: path to the CA cert. For a client this verifies the server certificate. Should
       only be used if `insecure` is set to true.
@@ -87,4 +89,4 @@ exporters:
 ```
 
 [beta]:https://github.com/open-telemetry/opentelemetry-collector#beta
-[contrib]:https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol-contrib
+[contrib]:https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol-contrib