Adding Mac-Only TLS information to ReadMe (#82)

Author: rccarper

README.md

@@ -16,13 +16,21 @@ to Java by the [aws-crt-java](https://github.com/awslabs/aws-crt-java) package.
 
 
 *__Jump To:__*
+* [Mac-Only TLS Behavior](#Mac-Only-TLS-Behavior)
 * [Installation](#Installation)
 * [Samples](samples)
 * [Getting Help](#Getting-Help)
 * [Giving Feedback and Contributions](#Giving-Feedback-and-Contributions)
 * [More Resources](#More-Resources)
 
 
+## 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.  When a stored private key from the Keychain is used, the following will be logged at the "info" log level:
+
+```
+static: certificate has an existing certificate-key pair that was previously imported into the Keychain.  Using key from Keychain instead of the one provided.
+```
 
 ## Installation
 
@@ -125,4 +133,4 @@ is provided by code that been generated from a model of the service.
 
 # License
 
-This library is licensed under the Apache 2.0 License.
+This library is licensed under the Apache 2.0 License.