Align SPM plugin docs with grpc-swift
Align SPM plugin docs with grpc-swift

# Motivation
We added some docs to clarify the paths of files in grpc-swift which we should state here as well.

thomasvl/swift-protobuf,apple/swift-protobuf,thomasvl/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,thomasvl/swift-protobuf,thomasvl/swift-protobuf,thomasvl/swift-protobuf,thomasvl/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,thomasvl/swift-protobuf,apple/swift-protobuf