Add macOS build badge and explain why it is failing.

kokoro/README.md

 # Tink presubmit tests and continuous integration
   
-![Kokoro Ubuntu](https://storage.googleapis.com/tink-kokoro-build-badges/tink.png)
-  
-Tink is testing continously with 
+![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 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.
+
+Presubmit tests are triggered for the pull request if one of the following
+conditions is met:
+
+ - The pull request is created by a Googler.
+ - The pull request is attached with a kokoro:run label.
 
- - Pull request is created by a Googler.
+Continuous builds are triggered for every new commit. Each continuous
+build on Ubuntu build the following Maven artifacts from HEAD of master:
 
- - Pull request is attached with a kokoro:run label.
+ - 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.