OSX TLS ReadMe Change (#169)

Adding messaging about Mac-Only TLS behavior.

Author: rccarper

README.md

@@ -24,3 +24,11 @@ python -m pip install .
 ````
 
 To use from your Python application, declare `awscrt` as a dependency in your `setup.py` file.
+
+## 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 v0.6.2, 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.
+```