docs(cc): explain how Conventional Commits format is used for issues and pull requests

[geoffreyvanwyk]

In Contributing section of README, explain that Conventional Commits format is not only used for commits but also for issues and pull requests.

The advantage of using it for issues is that the issue title and body can be copied into the commit message (with tweaks where necessary), making it less of a hassle to create the commit message.

The advantage of using it for pull requests is that GitHub's automatic release notes generator uses the titles of the pull requests since the previous tag. That way, the notes for a GitHub release matches the vision of the Conventional Commits project.