Mention protobuf in readme

README.md

@@ -4,10 +4,9 @@ Command line interface for [Golem OSS](https://golem.cloud).
 
 For Golem Cloud version client see [Golem Cloud CLI](https://github.com/golemcloud/golem-cloud-cli).
 
-
 ## Installation
 
-To install `golem-cli` you currently need to use `cargo`, Rust's build tool. 
+To install `golem-cli` you currently need to use `cargo`, Rust's build tool.
 
 To get `cargo` on your system, we recommend to use [rustup](https://rustup.rs/):
 
@@ -17,14 +16,23 @@ rustup install stable
 rustup default stable
 ```
 
+Another external dependency is `protobuf`, with a minimum version of `3.15`,
+which can be installed as described on http://google.github.io/proto-lens/installing-protoc.html. On macOS, you can install it with Homebrew:
+
+```shell
+brew install protobuf
+```
+
 Then you can install `golem-cli` with the following command:
 
 ```shell
 cargo install golem-cli
 ```
 
 ## More information
-Please check the [Golem Cloud developer documentation portal](https://learn.golem.cloud) to learn more about how to get started with *Golem Cloud*!
+
+Please check the [Golem Cloud developer documentation portal](https://learn.golem.cloud) to learn more about how to get started with _Golem Cloud_!
 
 ## Contributing
+
 Find details [here](CONTRIBUTING.md)