Add note to Mac installer ReadMe about macOS 10.15 Gatekeeper changes. (

pythonGH-18648)

Mac/BuildScript/resources/ReadMe.rtf

@@ -1,5 +1,5 @@
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-{\fonttbl\f0\fswiss\fcharset0 Helvetica;\f1\fswiss\fcharset0 Helvetica-Bold;\f2\fswiss\fcharset0 Helvetica-Oblique;
+{\rtf1\ansi\ansicpg1252\cocoartf2511
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 \f3\fmodern\fcharset0 CourierNewPSMT;}
 {\colortbl;\red255\green255\blue255;}
 {\*\expandedcolortbl;;}
@@ -56,6 +56,12 @@ Due to new security checks on macOS 10.15 Catalina, when launching IDLE macOS ma
 \f0\b0  button to proceed.\
 \
 
+\f1\b \ul macOS 10.15 (Catalina) Gatekeeper Requirements [changed in 3.9.0a4]\
+
+\f0\b0 \ulnone \
+As of 2020-02-03, Apple has changed how third-party installer packages, like those provided by python.org, are notarized for verification by Gatekeeper and begun enforcing additional requirements such as code signing and use of the hardened runtime.  As of 3.9.0a4, python.org installer packages now meet those additional notarization requirements.  The necessary changes in packaging should be transparent to your use of Python but, in the unlikely event that you encounter changes in behavior between 3.9.0a4 and earlier 3.9.0 alphas in areas like ctypes, importlib, or mmap, please check bugs.python.org for existing reports and, if necessary, open a new issue.\
+\
+
 \f1\b \ul Other changes\
 
 \f0\b0 \ulnone \