Adding Mac-only TLS ReadMe change (aws#153)

Author: rccarper

README.md

@@ -12,6 +12,14 @@ C++ wrapper around the aws-c-* libraries. Provides Cross-Platform Transport Prot
 
 More protocols and utilities are coming soon, so stay tuned.
 
+### 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.
+```
+
 ## Building
 
 The C99 libraries are already included for your convenience as submodules. If you would like to have us build them