[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

llvm-svn: 310700