Add OSS-Fuzz fuzzing badge (#278)

Adds based on instructions at
https://google.github.io/oss-fuzz/getting-started/new-project-guide/#status-badge

Patch By: Abhishek Arya (@inferno-chromium)

README.md

@@ -5,6 +5,7 @@ c-ares
 [![Windows Build Status](https://ci.appveyor.com/api/projects/status/03i7151772eq3wn3/branch/master?svg=true)](https://ci.appveyor.com/project/c-ares/c-ares)
 [![Coverage Status](https://coveralls.io/repos/c-ares/c-ares/badge.svg?branch=master&service=github)](https://coveralls.io/github/c-ares/c-ares?branch=master)
 [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/291/badge)](https://bestpractices.coreinfrastructure.org/projects/291)
+[![Fuzzing Status](https://oss-fuzz-build-logs.storage.googleapis.com/badges/c-ares.svg)](https://bugs.chromium.org/p/oss-fuzz/issues/list?sort=-opened&can=1&q=proj:c-ares)
 [![Releases](https://coderelease.io/badge/c-ares/c-ares)](https://coderelease.io/github/repository/c-ares/c-ares)
 
 This is c-ares, an asynchronous resolver library.  It is intended for