ticket:10740: Updated ICU4C readme.html for 53rc.

git-svn-id: http://source.icu-project.org/repos/icu/icu@35510 251d0590-4201-4cf1-90de-194747b24ca1
ticket:10740: ICU4C 53 maintenance branch copied from trunk r35510

git-svn-id: http://source.icu-project.org/repos/icu/icu@35511 251d0590-4201-4cf1-90de-194747b24ca1
ticket:10740: Tagged ICU4C maint-53 r35511 as release-53-rc

git-svn-id: http://source.icu-project.org/repos/icu/icu@35512 251d0590-4201-4cf1-90de-194747b24ca1