[Bash-autocompletion] Add --autocomplete flag to 5.0 release notes

Summary:
I thought we should add this information to release notes, because we
added a new flag to clang driver.

Reviewers: v.g.vassilev, teemperor, ruiu

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D36567

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@310700 91177308-0d34-0410-b5e6-96231b3b80d8