docs: list REFERENCES_FILE in the README edge types table

The full Edge Types list near the graph schema section still missed
the new REFERENCES_FILE type added by the doclinks pass. The
"selected" list earlier in the file is explicitly a subset and is
left as is.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Signed-off-by: Clément Deust <clement.deust@gmail.com>