Mention `scala_proto_register_toolchains` in docs

[luser]

Hi, thanks for this project! I was trying to get a simple example working with scala_proto_library and kept getting a no matching toolchains error from bazel. After reading the bazel docs on toolchains and this repository's WORKSPACE file I realized that I needed scala_proto_register_toolchains in my WORKSPACE and got things working. It would be great if the docs mentioned this!

[SrodriguezO]

@luser sorry we missed this! I'm glad you figured it out though. I'll update the docs later today to clarify this.

I haven't gotten to this; it's on my to-do list though. @borkaehw can you assign this task to me?