Add notes about zlib-ng

TODO.md

@@ -18,6 +18,27 @@ Contents
 
 - [HISTORICAL NOTES](#historical-notes)
 
+zlib-ng
+=======
+
+* Write ZLIB COMPATIBILITY section of README-maintainer.md.
+* Note: deterministic IDs are affected by choice of zlib
+
+```
+cd /tmp
+git clone https://github.com/zlib-ng/zlib-ng
+cd zlib-ng
+cmake -B build -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/tmp/inst -DZLIB_COMPAT=ON
+cmake --build build -j $(nproc)
+(cd build; ctest --verbose)
+cmake --install build
+```
+
+Then run qpdf's test suite with
+```
+LD_PRELOAD=/tmp/inst/lib/libz.so.1 ctest --verbose
+```
+
 Always
 ======