add link to new article in README

Author: glarregay-tob

program-analysis/echidna/advanced/README.md

@@ -9,3 +9,4 @@
 - [How to use hevm cheats to test permit](./hevm-cheats-to-test-permit.md): How to test code that depends on ecrecover signatures using hevm cheat codes
 - [How to seed Echidna with unit tests](./end-to-end-testing.md): How to use existing unit tests to seed Echidna
 - [Understanding and using `multi-abi`](./using-multi-abi.md): What is `multi-abi` testing, and how can it be used
+- [Interacting with off-chain data via FFI cheatcode](./interacting-with-offchain-data-via-ffi.md): Using the `ffi` cheatcode as a way of communicating with the operating system.