Add badge linking to generated Rust docs
[![Build Status](https://img.shields.io/badge/build-status-blue)](https://pantheon.corp.google.com/cloud-build/dashboard?project=oak-ci)
[![Fuzzing Status](https://oss-fuzz-build-logs.storage.googleapis.com/badges/oak.svg)](https://bugs.chromium.org/p/oss-fuzz/issues/list?sort=-opened&can=1&q=proj:oak)
+[![Docs](https://img.shields.io/badge/docs-rust-brightgreen)](https://project-oak.github.io/oak)
The goal of Project Oak is to create a specification and a reference
implementation for the secure transfer, storage and processing of data.
