Update XZ resources readme

Author: dwoz

relenv/_resources/xz/readme.md

@@ -1,4 +1,13 @@
-The config.h file was removed from XZ-Utils tarting with version 5.5.0.
-XZ-Utils seems to build just fine with the config.h file from 5.4.7, so we're
-including it here. This will be copied into the src/windows directory in the
-extracted source for XZ-Utils.
+The config.h file was removed from XZ-Utils starting with version 5.5.0.
+XZ-Utils switched to CMake and removed Visual Studio project files and
+pre-generated headers/sources.
+
+We include the following files from XZ 5.4.7 to maintain compatibility with
+Python's MSBuild-based build system on Windows (PCbuild/liblzma.vcxproj):
+
+- config.h (src/common/config.h)
+- crc32_table.c (src/liblzma/check/crc32_table.c)
+- crc64_table.c (src/liblzma/check/crc64_table.c)
+
+These files are copied into the extracted XZ source directory during the
+Windows build process if they are missing.