doc: describe how to update zlib

See:
- nodejs#31201

Author: sam-github

doc/guides/maintaining-zlib.md

@@ -1,6 +1,34 @@
-This copy of zlib comes from the Chromium team's zlib fork which incorporated performance improvements not currently available in standard zlib.
+# Maintaining zlib
 
-To update this code:
+This copy of zlib comes from the Chromium team's zlib fork which incorporated
+performance improvements not currently available in standard zlib.
 
-* Clone https://chromium.googlesource.com/chromium/src/third_party/zlib
-* Comment out the `#include "chromeconf.h"` in zconf.h to maintain full compatibility with node addons
+## Updating zlib
+
+Update zlib:
+```shell
+git clone https://chromium.googlesource.com/chromium/src/third_party/zlib
+cp deps/zlib/zlib.gyp deps/zlib/win32/zlib.def deps
+rm -rf deps/zlib zlib/.git
+mv zlib deps/
+mv deps/zlib.gyp deps/zlib/
+mkdir deps/zlib/win32
+mv deps/zlib.def deps/zlib/win32
+sed -i -- 's_^#include "chromeconf.h"_//#include "chromeconf.h"_' deps/zlib/zconf.h
+```
+
+Check that Node.js still builds and tests.
+
+It may be necessary to update deps/zlib/zlib.gyp if any significant changes have
+occurred upstream.
+
+## Commiting zlib
+
+Add zlib: `git add --all deps/zlib`
+
+Commit the changes with a message like
+```text
+deps: update zlib to upstream d7f3ca9
+
+Updated as described in doc/guides/maintaining-zlib.md.
+```