Add more about Apple silicon builds

README.rst

@@ -431,6 +431,34 @@ To use these scripts
 * Make sure your project is set up to build on AppVeyor, and you should now
   be ready (for what could be another round of slow debugging).
 
+* For Apple silicon support you can either create an ``arm64`` wheel or
+  a ``universal2`` wheel by supplying ``PLAT`` env variable.
+  ``universal2`` builds work on both ``arm64`` and ``x86_64`` platforms
+  and also allows the ability to switch the architecture on ``universal2``
+  python builds on Apple silicon machines where ``x86_64`` can be run
+  using Rosetta2 emulation.
+
+  There are two ways to build ``universal2`` builds.
+  1. Build with ``-arch x86_64 -arch arm64``.
+     These flags instruct the C/C++ compiler to compile twice and create a
+     fat object/executable/library. This is the easiest, but have several
+     drawbacks. If you are using C/C++ libraries that are built using
+     library_builders, it's highly likely that they don't build correctly
+     because most build systems and packages don't support building fat binaries.
+     We could possibly build them separately and fuse them, but the headers might
+     not be identical which is required when building the wheel as a ``universal2``
+     wheel. If you are using Fortran, ``gfortran`` doesn't support fat binaries.
+
+  2. Build ``arm64`` and ``x86_64`` wheels seperately and fuse them.
+     For this to work, we need to build the C/C++ libraries twice. Therefore,
+     the library building is once called with ``BUILD_PREFIX=${BUILD_PREFIX:-/usr/local}``
+     for ``x86_64`` and then called again with ``BUILD_PREFIX=/opt/arm64-builds``.
+     Once the two wheels are created, these two are merged. Both the
+     ``arm64`` and ``universal2`` wheels are outputs for this build.
+
+  To build Apple silicon builds, you should use a CI service with Xcode 12 with
+  universal build support and make sure that xcode is the default.
+
 If your project depends on NumPy, you will want to build against the earliest
 NumPy that your project supports - see `forward, backward NumPy compatibility
 <https://stackoverflow.com/questions/17709641/valueerror-numpy-dtype-has-the-wrong-size-try-recompiling/18369312#18369312>`_.