[docs] LLVM Security Group and Process

Summary:
See the corresponding RFC on llvm-dev for a discussion of this proposal.
  http://lists.llvm.org/pipermail/llvm-dev/2019-November/136839.html

Subscribers: jkorous, dexonsmith, arphaman, ributzka, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70326