Fix table of contents of `CONTRIBUTING.md`

[davidbtadokoro]

CONTRIBUTING.md has two nonexistent sections (Coding Style and Communication), a missing section (License), and a subsection (Development Workflow). We should:

1. Remove the nonexistent sections;
2. Add the missing section;
3. Point to the section instead of the subsection.

[theavitw]

Can I take this issue?

[abhilov23]

Is it still up? I would like to work on this.