minor(docs): Add docs for regen.sh (apache#13321)

docs/source/contributor-guide/howtos.md

@@ -132,3 +132,17 @@ After you've confirmed your `taplo` version, you can format all the `.toml` file
 ```bash
 taplo fmt
 ```
+
+## How to update protobuf/gen dependencies
+
+The prost/tonic code can be generated by running `./regen.sh`, which in turn invokes the Rust binary located in [gen](./gen)
+
+This is necessary after modifying the protobuf definitions or altering the dependencies of [gen](./gen), and requires a
+valid installation of [protoc] (see [installation instructions] for details).
+
+```bash
+./regen.sh
+```
+
+[protoc]: https://github.com/protocolbuffers/protobuf#protocol-compiler-installation
+[installation instructions]: https://datafusion.apache.org/contributor-guide/getting_started.html#protoc-installation