Adding a HowTo for Attributes.

This is based on Bill Wendling's email.  No additional content has been added,
but now there's a place for Attributes to capture future information.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174961 91177308-0d34-0410-b5e6-96231b3b80d8