COFF: Update README.
COFF: Update README.

It's no longer "new" because the old COFF linker has been removed.
Update the introduction accordingly.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@244565 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld