README: add an OSS-Fuzz badge

Since bug reports, backtraces, coverage reports and build logs are scattered
across at least four different places and there is no publicly available dashboards
the badge can point to, let's just point it to the build logs, which hopefully are going to be
a little bit more usable once https://github.com/google/oss-fuzz/issues/2690 is
addressed.

README.md

@@ -5,6 +5,7 @@
 [![Semaphore CI Build Status](https://semaphoreci.com/api/v1/projects/28a5a3ca-3c56-4078-8b5e-7ed6ef912e14/443470/shields_badge.svg)](https://semaphoreci.com/systemd/systemd)<br/>
 [![Coverity Scan Status](https://scan.coverity.com/projects/350/badge.svg)](https://scan.coverity.com/projects/350)<br/>
 [![Fuzzit Status](https://app.fuzzit.dev/badge?org_id=systemd&branch=master)](https://app.fuzzit.dev/orgs/systemd/dashboard)<br/>
+[![OSS-Fuzz Status](https://oss-fuzz-build-logs.storage.googleapis.com/badges/systemd.svg)](https://oss-fuzz-build-logs.storage.googleapis.com/index.html)<br/>
 [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/1369/badge)](https://bestpractices.coreinfrastructure.org/projects/1369)<br/>
 [![Travis CI Build Status](https://travis-ci.org/systemd/systemd.svg?branch=master)](https://travis-ci.org/systemd/systemd)<br/>
 [![Language Grade: C/C++](https://img.shields.io/lgtm/grade/cpp/g/systemd/systemd.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/systemd/systemd/context:cpp)<br/>