Update README.md with additional instructions for protoc and compilation

[ronnikc]

Updated gRPC API with additional instructions and information for protoc and compilation.

Purpose

Missing instructions and detail to support developers in utilizing the gRPC API repo.

Changes

Added instructions and information for protoc and compilation.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.

CLA acceptance

_Remove if not applicable.

By submitting the contribution I accept the terms and conditions of the
Contributor License Agreement v1.0

• link: https://developers.concordium.com/CLAs/Contributor-License-Agreement-v1.0.pdf

• I accept the above linked CLA.