Document update-protos requiring python2
Not actually sure if it strictly requires Python 2 or what, but that's what fixed it for me.
There may be other requirements that aren't obvious to me (e.g., is the Go compiler
needed?).
@@ -38,7 +38,7 @@ If you modify a .proto file, you'll also need to generate and check in the
.pb.go files.
Run `bazel run //hack:update-protos` to generate, and `bazel run //hack:verify-protos.sh`
-to verify.
+to verify. This command requires `python` to be installed.
## Testing
