docs: #556

- improvement to gcp build docs

docs/unicodejsps/gcp-run.md

@@ -12,10 +12,12 @@
 mvn -B package -am -pl UnicodeJsps -DskipTests=true
 ```
 
-- make a copy of CLDR - lots of ways to do this
+- make a copy of CLDR - lots of ways to do this. `--reference-if-able ~/src/cldr` is another directory on my disk which has a copy of CLDR, to save copying. The CLDR_REF calculation is to make sure you have the same CLDR version as the build.
 
 ```
-git clone --reference-if-able ~/src/cldr https://github.com/unicode-org/cldr.git
+git clone https://github.com/unicode-org/cldr.git     --reference-if-able ~/src/cldr
+CLDR_REF=$(mvn help:evaluate -Dexpression=cldr.version -q -DforceStdout | cut -d- -f3)
+(cd cldr ; git reset --hard ${CLDR_REF})
 mkdir -p UnicodeJsps/target && tar -cpz --exclude=.git -f UnicodeJsps/target/cldr-unicodetools.tgz ./cldr/ ./unicodetools/
 ```