Add small excerpt about scope and breaking changes of Conventional Commits to `CONTRIBUTING.md`

[davidbtadokoro]

In the Use Conventional Commits sections, there is no mention of the use of the scope and breaking changes syntax. For example, for a breaking feature that is specific to the foo module, the commit message should have a format like

feat(foo)!: introduce breaking change bar

I think this notion should be introduced really briefly with maybe one line of description and one line of example. I think a good reference is this.

[ivinjabraham]

I believe this is closed after #144?