Add note about prerequisites to CONTRIBUTING.md

[morigs]

As a new contributor I had some problems setting up opentelemetry-rust.
opentelemetry-otlp requires following:

• protoc
• cmake
• llvm (+ LIBCLANG_PATH env)

I think this should be described in the CONTRIBUTING.md. I can submit a PR

[jtescher]

@morigs sounds good! It is unfortunate that the grpc requirements have to be set up to work on any project in this workspace.