Add Ethernaut Level 15 (NaughtCoin) as an example

The [blog post](https://wizzardhat.com/ethernaut-level-15-naughtcoin/) by @ChmielewskiKamil about writting Echidna properties for NaughtCoin is great. Are you interested in creating a PR to keep it as one example?

Start with the linked NaughtCoin blog post and inspect the repository's existing examples to find the appropriate location and format. Done means adding Ethernaut Level 15 as a consistent example covering the NaughtCoin Echidna properties described in the post.

Written by the indexing model from the issue text.