document the reason

Author: masci

appveyor.yml

@@ -26,7 +26,8 @@ install:
   # Python dependencies needed at test time
   - python -V
   - pip install -r test\requirements.txt
-  # protobuf tooling needed at test time
+  # protobuf tooling needed at test time. We use a very old version of the compiler
+  # because of this: https://github.com/protocolbuffers/protobuf/issues/3957
   - go get github.com/golang/protobuf/protoc-gen-go
   - curl -o protoc.zip -LO https://github.com/protocolbuffers/protobuf/releases/download/v3.4.0/protoc-3.4.0-win32.zip
   - 7z e protoc.zip -o%PROTOC_PATH%