[readme] add oss-fuzz badge

Related:
https://github.com/google/oss-fuzz/pull/2513
https://github.com/systemd/systemd/commit/ce2098b7e9443cd6f31fb70af7f72308cd2962a3

README.md

 [![Travis Build Status](https://travis-ci.org/harfbuzz/harfbuzz.svg?branch=master)](https://travis-ci.org/harfbuzz/harfbuzz)
 [![AppVeyor Build Status](https://ci.appveyor.com/api/projects/status/0t0flrxpstj9lb9w?svg=true&branch=master)](https://ci.appveyor.com/project/harfbuzz/harfbuzz)
 [![CircleCI Build Status](https://circleci.com/gh/harfbuzz/harfbuzz/tree/master.svg?style=svg)](https://circleci.com/gh/harfbuzz/harfbuzz/tree/master)
+[![OSS-Fuzz Status](https://oss-fuzz-build-logs.storage.googleapis.com/badges/harfbuzz.svg)](https://oss-fuzz-build-logs.storage.googleapis.com/index.html)
 [![Coverity Code Health](https://img.shields.io/coverity/scan/5450.svg)](https://scan.coverity.com/projects/behdad-harfbuzz)
 [![Codacy Code Health](https://api.codacy.com/project/badge/Grade/f17f1708783c447488bc8dd317150eaa)](https://app.codacy.com/app/behdad/harfbuzz)
 [![Codecov Code Coverage](https://codecov.io/gh/harfbuzz/harfbuzz/branch/master/graph/badge.svg)](https://codecov.io/gh/harfbuzz/harfbuzz)