README.md: Added status badge for OSS-Fuzz

OSS-Fuzz [1] is a decentralized fuzzing service for open-source
software projects. This badge shows the status of libcoap wrt to the
latest fuzzer runs.

[1] https://google.github.io/oss-fuzz/

README.md

@@ -2,6 +2,7 @@
 
 [![Build Status](https://travis-ci.org/obgm/libcoap.svg?branch=master)](https://travis-ci.org/obgm/libcoap)
 [![Static Analysis](https://scan.coverity.com/projects/10970/badge.svg?flat=1)](https://scan.coverity.com/projects/obgm-libcoap)
+[![Fuzzing Status](https://oss-fuzz-build-logs.storage.googleapis.com/badges/libcoap.svg)](https://bugs.chromium.org/p/oss-fuzz/issues/list?sort=-opened&can=1&q=proj:libcoap)
 
 Copyright (C) 2010—2019 by Olaf Bergmann <bergmann@tzi.org> and others