Added a summary description of ISO week based date, and more descriptive comments

git-svn-id: http://svn.savannah.nongnu.org/svn/avr-libc/trunk@2394 1084037a-409f-4a99-8160-1a4b3577d4c1
A blurb in the News about the time package

git-svn-id: http://svn.savannah.nongnu.org/svn/avr-libc/trunk@2395 1084037a-409f-4a99-8160-1a4b3577d4c1