Fix protobuf

* Rebuiild with protoc v3.17.2 and protoc-gen-go v1.26.0
* Add misisng go_package flag
* Add protobuf documentation

docs/protobuf.md

@@ -0,0 +1,34 @@
+# Protobuf
+
+The `.proto` files contains a list of fields that are populated by GoFlow2.
+
+If the fields are changed, the schema needs to be recompiled
+in order to use it.
+
+The compilation is dependent on the language.
+Keep in mind the protobuf source code and libraries changes often and this page may be outdated.
+
+For other languages, refer to the [official guide](https://developers.google.com/protocol-buffers).
+
+## Compile for Golang
+
+The following two tools are required:
+* [protoc](https://github.com/protocolbuffers/protobuf), a protobuf compiler, written in C
+* [protoc-gen-go](https://github.com/protocolbuffers/protobuf-go), a Go plugin for protoc that can compile protobuf for Golang
+
+The release page in the respective GitHub repositories should provide binaries distributions. Unzip/Untar if necessary.
+Make sure that the two binaries are in your ``$PATH``. On Mac OS you can add the files to `/usr/local/bin` for instance.
+
+From the root of the repository, run the following command:
+
+```bash
+$ protoc --go_opt=paths=source_relative --go_out=. pb/*.proto
+```
+
+This will compile the main protobuf schema into the `pb` directory.
+
+You can also run the command which will also compile the protobuf for the sample enricher.
+
+```bash
+$ make proto
+```