doc: document ICU time zone data update process

Updates the "Maintaining ICU" document and describes the process
to update an existing ICU `.dat` file with updated binary time
zone data files corresponding to new IANA `tzdata` releases.

Requested in #30211 by @srl295

PR-URL: #30364
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Steven R Loomis <srloomis@us.ibm.com>

doc/guides/maintaining-icu.md

@@ -33,6 +33,41 @@ $ node -p process.versions
 }
 ```
 
+### Time Zone Data
+
+Time zone data files are updated independently of ICU CLDR data.  ICU and its
+main data files do not need to be upgraded in order to apply time zone data file
+fixes.
+
+The [IANA tzdata](https://www.iana.org/time-zones) project releases new versions
+and announces them on the [`tz-announce`](http://mm.icann.org/pipermail/tz-announce/)
+mailing list.
+
+The Unicode project takes new releases and publishes
+[updated time zone data files](https://github.com/unicode-org/icu-data/tree/master/tzdata/icunew)
+in the icu/icu-data repository.
+
+All modern versions of Node.js use the version 44 ABI of the time zone data
+files.
+
+#### Example: Updating the ICU `.dat` File
+
+* Decompress `deps/icu/source/data/in/icudt##l.dat.bz2`, where `##` is
+  the ICU major version number.
+* Clone the icu/icu-data repository and copy the latest `tzdata` release `le`
+  files into the `source/data/in` directory.
+* Follow the upstream [ICU instructions](http://userguide.icu-project.org/datetime/timezone)
+  to patch the ICU `.dat` file:
+  > `for i in zoneinfo64.res windowsZones.res timezoneTypes.res metaZones.res;
+  > do icupkg -a $i icudt*l.dat`
+* Optionally, verify that there is only one of the above files listed when using
+  `icupkg -l`.
+* Optionally, extract each file using `icupkg -x` and verify the `shasum`
+  matches the desired value.
+* Compress the `.dat` file with the same filename as in the first step.
+* Build, test, verifying `process.versions.tz` matches the desired version.
+* Create a new minor version release.
+
 ## Release Schedule
 
 ICU typically has >1 release a year, particularly coinciding with a major