[WIP] RocketChip Code Style contract

RocketChip should be added some documentations to unify the code style:

**Type of issue**: feature request

**Impact**: no functional change

**Development Phase**: proposal

**Other information**
https://github.com/chipsalliance/rocket-chip/issues/255

**What is the use case for changing the behavior?**

Start by reading the linked RocketChip issue #255 and inspecting the repository for existing code-style documentation. Clarify which Scala and RocketChip conventions the contract must cover; done means a documented, agreed style guide with no functional changes.

Written by the indexing model from the issue text.