Add an example for Check-Effects-Interactions & FREI-PI

[andrejrakic]

Hello! First of all, thank you for creating and maintaining this extremely valuable list of common Solidity patterns, personally that's something I was looking for (for a while 😄).

Secondly, in my opinion, it will be useful to add the latest Solidity pattern: "Function Requirements-Effects-Interactions + Protocol Invariants", proposed by @brockelmore in the following article.

Thanks in advance for your consideration.

[merklejerk]

Yes! I agree, these should be included. Stay tuned.