Quickbook: Backdated changelog for 1.5.8.

Oops, I completely forgot to write this up at the time.

git-svn-id: https://svn.boost.org/svn/boost/trunk/tools/quickbook@82573 b8fc166d-592f-0410-95f2-cb63ce0dd405
Merge quickbook 1.5.8 release notes from 'trunk'
Bump version number.