[docs] Update status.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@172501 91177308-0d34-0410-b5e6-96231b3b80d8
[docs] Add driver documentation.

This gives an overview of the driver and explains how to add options and driver flavors.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@172502 91177308-0d34-0410-b5e6-96231b3b80d8