Update luhn description per suggestion.

See https://github.com/exercism/exercism.io/issues/1929
Expand on the checksum idea for luhn
Better formatting and re-organised content

The description was hard to parse for non-native-English speakers. Also a little hard to understand for beginner programmers.
Merge pull request #33 from hrj/patch-1

Better formatting and re-organised content
Fix typo
Fix missing space between words
Merge pull request #34 from locks/patch-1

Improves dominoes.md
Merge pull request #32 from soniakeys/test-data

clock.json: test data for clock
update README.md with .json doc.