Add Turbo Cache as a server implementation (bazelbuild#231)

Author: allada

README.md

@@ -61,6 +61,7 @@ workers implement the Remote Worker API.
 * [Flare Build Execution](https://flare.build/products/flare-build-execution) (commercial)
 * [Remote Build Execution (Alpha)](https://blog.bazel.build/2018/10/05/remote-build-execution.html) (commercial)
 * [Scoot](https://github.com/twitter/scoot) (open source)
+* [Turbo Cache](https://github.com/allada/turbo-cache) (open source)
 
 ## API Community