[Doc] Guideline on adding exception handling support for a target

Summary:
This is the first attempt to write down a guideline on adding exception handling support for a target. The content basically bases on the discussion on [1]. If you guys know who is exception handling expert, please add him as the reviewer. Thanks.

[1] http://lists.llvm.org/pipermail/llvm-dev/2018-January/120405.html

Reviewers: t.p.northover, theraven, nemanjai

Reviewed By: theraven

Subscribers: sdardis, llvm-commits

Differential Revision: https://reviews.llvm.org/D42178

llvm-svn: 323311