Define sysl GRPC representation

[andrewemeryanz]

In order to build out grpc support within sysl, an authoritative document needs to exist that details how a grpc specification is represented in sysl.

Current work within this area includes the protoc-gen-sysl plugin (a protoc plugin to generate sysl from a grpc specification) and some early work to generate a grpc specification from sysl.

The document must detail at least the following:

1. Which grpc features can be represented in sysl and to what extent.
2. How supported grpc features are represented in sysl.
3. What transformations are necessary (such as the renaming of identifiers) to support translation between formats.

[andrewemeryanz]

Linked issues:
grpc to sysl: anz-bank/sysl-go#98
sysl to grpc: #847

[joshcarp]

Found here: https://github.com/anz-bank/protoc-gen-sysl#mapping-from-proto-to-sysl