Add some readme files, start fixing docker build.

Docker (and CI in general) build currently isn't generating protobuf files.
Docker image also needs to check that pigpio works.

protobuf/README.md

@@ -0,0 +1,11 @@
+# Protobuf
+
+This module contains the protobuf definitions used by PiCar.
+
+## Build
+
+Run the gradle task
+
+`./gradlew generateProto`
+
+It's not recommended to run this alone, unless you plan on manually copying the generated files. The build.gradle files in the other projects will automatically copy the correct generated code across, which is recommended to use instead.