Document protobuf code generation and output requirements

[pharr117]

The cosmos-indexer-modules repo MUST provide packages that are compatible with inclusion in the Cosmos Indexer repo. This will require some of the following:

1. Pulling in proto type definitions, cleaning them up to only include messages of interest, and generating the correct code
2. Maintaining the proper versioning of the direct/indirect dependencies required by the Cosmos Indexer and thus the Cosmos Indexer Modules - if the dependencies list does not match, the packages will not be compatible
3. Finalizing the message inclusion so that they satisfy the Cosmos SDK Msg interface