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

yast/ycp-killer,yast/ycp-killer