ticket:11278: Update readme for GA

git-svn-id: http://source.icu-project.org/repos/icu/icu@36576 251d0590-4201-4cf1-90de-194747b24ca1
ticket:11278: Merge readme for GA into ICU4C maint-54 branch

git-svn-id: http://source.icu-project.org/repos/icu/icu@36577 251d0590-4201-4cf1-90de-194747b24ca1
ticket:11278: Reverted spoof data updates (#11262) merged from trunk in a previous commit.

git-svn-id: http://source.icu-project.org/repos/icu/icu@36578 251d0590-4201-4cf1-90de-194747b24ca1