Add Checkpoint

[bonustrack]

• Add "Checkpoint", a library for indexing StarkNet on-chain events, more infos here: http://checkpoint.fyi & https://github.com/snapshot-labs/checkpoint.
• Fixed a small typo on "starknet-scaffold" description.

Checklist

• [x] The URL is not already present in the list (check with CTRL/CMD+F in the raw markdown file).
• [x] Each description starts with an uppercase character and ends with a period.
  Example: cairo - The Cairo programming language.
• [x] Drop all A / An prefixes at the start of the description.
• [x] Avoid using the word StarkNet in the description.

[JorikSchellekens]

Checkpoint added in #55. Scaffold was omitted because it's out of date. Reopen if otherwise.