README: updating with schema information and updating instructions (#353)

* README: added preliminary section on schema processing

* Added text for adding ICU versions and new components

* Adding preliminary information about including new platorms

* Adding preliminary information about including new platorms

* Minor fix

* Add instructions for installing and running DDT

* Rearrange README to highlight running it. Add text on log files

* Fixing typos and reorganizing some things

* Fixing typos and rewriting some parts

* Update README.md

Co-authored-by: Elango Cheran <elango@unicode.org>

* Apply suggestions from code review

Co-authored-by: Elango Cheran <elango@unicode.org>

* Updating format

* Minor fix

* OK

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Co-authored-by: Elango Cheran <elango@unicode.org>

Author: Craig Cornelius