README should point to new post-re-organisation JSON locations

[petertseng]

README currently has:

Each problem can have a file <problem>.json

Rendered, this just shows up as "Each problem can have a file .json", but that's a separate issue.

The more important thing is that this should get changed to match the location after #351 moved to per-exercise directories.

[kytrinyx]

Good catch, fixing that now.