document updated icu4j build process

Notes:

1. ICU4J currently depends on ICU4C for its runtime data, and some (portable) test data.

2. Current documentation is located in the ICU4J readme under Building ICU4J Resources from ICU4C, and in a comment at the top of icu4c/source/tools/genrb/Makefile.in - SIMPLIFIED WAY TO BUILD ICU4J DATA FROM C

3. The build process starts with having ICU4C built normally, as well as icu4j

4. The three items that are built by ICU4C are currently: icudata.jar, testdata.jar, and icu4j/src/com/ibm/icu/dev/test/util/DebugUtilitiesData.java

5. DebugUtilitiesData contains exported enum types from C. ( it is a parallel to unicode/dbgutil.h in the C ctestfw package). - it is generated by having gentest called.

So, this task consists of reading the comment in the genrb Makefile.in, as well as the appropriate section of icu4j's readme, and updating both as appropriate.

One good way to do this might be to create an icu4j-readme.txt in icu4c/source/data/icu4j-readme.txt - and have the icu4j readme, and the comment in the Makefile.in, point to it. The part that deals with C tools should probably live on the C side, so that it stays in sync with the C tools.

Created the file icu4j-readme.txt in $icu4c_root/source/data

This file contains the new instructions for using the Makefile in source/tools/genrb, with special notes about Cygwin, AND the old instructions, if the new Makefile fails to work properly.

Updated $icu4j_root/readme.html to point to the new file "icu4j-readme.txt" Updated $icu4c_root/source/tools/genrb/Makefile.in to point to the new file "icu4j-readme.txt"

Pending approval by Steven and possible commit to new Cygwin branch.

Committed new docs. Set for review