add protobuf-compiler package to documentation

Initial title:

package "protobuf-compiler" was missing from this documentation; resulting in failed "cargo build"

Need the extra "protobuf-compiler" package as otherwise cargo build ends with:

...
   Compiling netlink-proto v0.10.0
   Compiling cid v0.10.1
   Compiling x509-parser v0.16.0
error: failed to run custom build command for `litep2p v0.6.2`

Caused by:
  process didn't exit successfully: `/home/ubuntu/development/git/duniter-v2s/target/debug/build/litep2p-dbc6ce967c7ab115/build-script-build` (exit status: 101)
  --- stderr
  thread 'main' panicked at /home/ubuntu/.cargo/registry/src/index.crates.io-6f17d22bba15001f/prost-build-0.11.9/src/lib.rs:1457:10:
  Could not find `protoc` installation and this build crate cannot proceed without
      this knowledge. If `protoc` is installed and this crate had trouble finding
      it, you can set the `PROTOC` environment variable with the specific path to your
      installed `protoc` binary.If you're on debian, try `apt-get install protobuf-compiler` or download it from https://github.com/protocolbuffers/protobuf/releases

  For more information: https://docs.rs/prost-build/#sourcing-protoc

  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
warning: build failed, waiting for other jobs to finish...

(was already correct in ./setup.md)