Add build instructions

Author: BCSharp

tests/ctypes_test/Makefile

@@ -1,3 +1,15 @@
+# By default, `make` produces a PYD for the current OS/processor.
+# To build a specific PYD, the full command is:
+#   * _ctypes_test_darwin_arm64.pyd:   make OS=darwin MACHINE=arm64
+#   * _ctypes_test_darwin_x86_64.pyd:  make OS=darwin MACHINE=x86_64
+#   * _ctypes_test_linux_aarch64.pyd:  make OS=linux MACHINE=aarch64
+#   * _ctypes_test_linux_x86_64.pyd:   make OS=linux MACHINE=x86_64 BITS=64
+#   * _ctypes_test_linux_i686.pyd:     make OS=linux MACHINE=i686 BITS=32
+# Parameter OS=... may be skipped if it is the same as the host OS.
+# For Windows builds, use `_ctypes_test.vcxproj` or `_ctypes_test.sln` with MSBuild:
+#   * _ctypes_test_win64.pyd:          msbuild -p:Configuration=Release -p:Platform=x64
+#   * _ctypes_test_win32.pyd:          msbuild -p:Configuration=Release -p:Platform=x86
+
 OS := $(shell uname -s | tr '[:upper:]' '[:lower:]')
 MACHINE ?= $(shell uname -m)
 BITS ?= 64