Add grpc URL example to the remote caching doc.
Closes
@@ -250,6 +250,7 @@ Use the following flag to read from and write to the remote cache:
```
build --remote_cache=http://replace-with-your.host:port
```
+Besides http, the following protocols are also supported: https, grpc, grpcs.
Use the following flag in addition to the one above to only read from the
remote cache:
