Add arm64 and universal2 to README

README.rst

@@ -272,6 +272,9 @@ To use these scripts
     # and test containers, via docker.
     dist: xenial
 
+    # osx image that enables Apple silicon builds
+    osx_image: xcode12u
+
     matrix:
       include:
         - os: linux
@@ -326,6 +329,10 @@ To use these scripts
         - os: osx
           env:
             - MB_PYTHON_VERSION=3.8
+        - os: osx
+          env:
+            - MB_PYTHON_VERSION=3.9
+            - PLAT="universal2"
         - os: osx
           env:
             - MB_PYTHON_VERSION=3.9
@@ -335,6 +342,11 @@ To use these scripts
             - MB_PYTHON_VERSION=pypy-5.7
 
     before_install:
+        # Setup xcode and compilers to use the new SDK for universal2 and arm64 builds
+        - if [ "$PLAT" == "universal2" ] || [ "$PLAT" == "arm64" ]; then
+              sudo xcode-select -switch /Applications/Xcode_12.2.app fi
+              export SDKROOT=/Applications/Xcode_12.2.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.0.sdk
+          fi
         - source multibuild/common_utils.sh
         - source multibuild/travis_steps.sh
         - before_install