message: Add macOS build badge and explain why it is failing.
diff: # Tink presubmit tests and continuous integration
  -![Kokoro Ubuntu](https://storage.googleapis.com/tink-kokoro-build-badges/tink.png)
  +![Kokoro Ubuntu](https://storage.googleapis.com/tink-kokoro-build-badges/tink-ubuntu.png)
  +![Kokoro macOS](https://storage.googleapis.com/tink-kokoro-build-badges/tink-macos.png)
  -Tink is testing continously with
  +Tink is testing continuously with
  [Kokoro](https://www.cloudbees.com/sites/default/files/2016-jenkins-world-jenkins_inside_google.pdf)
  an internal deployment of Jenkins at Google.
  -A presubmit build will be triggered for the pull request if one of the following conditions is met:
  +Continuous builds and presubmit tests are run on Ubuntu and macOS. As of June 15,
  +2017 macOS tests and builds are intentionally failing, because of some internal
  +issue with Kokoro.
  - - Pull request is created by a Googler.
  +Presubmit tests are triggered for the pull request if one of the following
  +conditions is met:
  - - Pull request is attached with a kokoro:run label.
  + - The pull request is created by a Googler.
  + - The pull request is attached with a kokoro:run label.
  +
  +Continuous builds are triggered for every new commit. Each continuous
  +build on Ubuntu build the following Maven artifacts from HEAD of master:
  +
  + - https://storage.googleapis.com/tink-build-artifacts/maven/tink-1.0.0-SNAPSHOT.jar
  + - https://storage.googleapis.com/tink-build-artifacts/maven/tink-1.0.0-SNAPSHOT-sources.jar
  + - https://storage.googleapis.com/tink-build-artifacts/maven/tink-1.0.0-SNAPSHOT-javadoc.jar
  -Continuous and presubmit builds are done on Ubuntu and macOS.
  