Document libFuzzer on Windows.

Summary:
Document that libFuzzer supports Windows, how to get it,
and its limitations.

Reviewers: kcc, morehouse, rnk, metzman

Reviewed By: kcc, rnk, metzman

Subscribers: hans, rnk

Differential Revision: https://reviews.llvm.org/D57597

llvm-svn: 353551