Allow keg root README.md more than strict KEGML

[rwxrob]

Since main entry into KEG Site need to be able to organize different content paths and entry points into the overall keg. A node needs more strictness to avoid creating nodes that are too large. But this doesn't apply to the keg itself, which benefits from an exhaustive main entry point.

• Allow multiple headings
• Emphasize linking into anything underneath
• Node includes become single integers with query codes (no ../ prefix required)