README.md: Make "Module Metadata" a level 2 heading

It doesn't really belong to the "Usage" section. This change also puts
the "Module Metadata" section to the same level as "New YaST Module
Structure", which is kind of similar (both are essentially references).
README.md: Reorder command descriptions alphabetically
README.md: Add stubs for "yk build" and "yk submit"

Content will be filled-in later.
README.md: Add "Contents" section
Merge pull request #388 from yast/readme-structural-improvements

[Review] Request from '' @ 'yast/ycp-killer/readme-structural-improvements'