Document system requirements to use Lagom's protobuf tasks

Lagom’s build includes some protobuf compilation tasks and settings. The requirements to run these tasks successfully are undocumented.

(at least) the following are necessary:

• protoc of the correct version in the PATH
• more?

[erip]

@ignasi35 To be clear, using sbt-protobuf and protoc-jar aren’t mutually exclusive. In fact, I think they’re quite complementary. sbt-protobuf simplifies generating sources given .proto files and an arbitrary protoc implementation; protoc-jar provides a protoc implementation.

By combining them, there’s no long a strict dependency that protoc is installed on a machine.

[ignasi35]

Until we change how to run protoc from sbt, the steps required are:

• install protoc locally and setup the PATH (see comments above for details wrt versions)
• sbt ;protobufGenerate;javafmt