Add fuzzing badge in readme
From
[![Go Report Card](https://goreportcard.com/badge/github.com/prometheus/prometheus)](https://goreportcard.com/report/github.com/prometheus/prometheus)
[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/486/badge)](https://bestpractices.coreinfrastructure.org/projects/486)
[![Gitpod ready-to-code](https://img.shields.io/badge/Gitpod-ready--to--code-blue?logo=gitpod)](https://gitpod.io/#https://github.com/prometheus/prometheus)
+[![Fuzzing Status](https://oss-fuzz-build-logs.storage.googleapis.com/badges/prometheus.svg)](https://bugs.chromium.org/p/oss-fuzz/issues/list?sort=-opened&can=1&q=proj:prometheus)
Visit [prometheus.io](https://prometheus.io) for the full documentation,
examples and guides.
