Updated readme.txt

Author: zooba

PCbuild/readme.txt

@@ -252,6 +252,16 @@ _tkinter
     Win32 and "tcltk64" on x64.  They also copy the Tcl and Tk DLLs
     into the current output directory, which should ensure that Tkinter
     is able to load Tcl/Tk without having to change your PATH.
+zlib-ng
+    Compiles zlib-ng as a static library, which is later included by
+    pythoncore.vcxproj. This was generated using CMake against zlib-ng
+    version 2.2.4, and should be minimally updated as needed to adapt
+    to changes in their source layout. The zbuild.h, zconf.h and
+    zconf-ng.h file in the PC directory were likewise generated and
+    vendored.
+
+    Sources for zlib-ng are imported unmodified into our source
+    repository at https://github.com/python/cpython-source-deps.
 
 
 Getting External Sources