Add tests/ui/ffi section to tests UI README

Author: reddevilmidzy

tests/ui/README.md

@@ -556,6 +556,12 @@ Tests for the `--extern` CLI flag.
 
 Tests on feature-gating, and the `#![feature(..)]` mechanism itself.
 
+## `tests/ui/ffi/`: Foreign Function Interface
+
+Tests for the `std::ffi` module.
+
+See [`std::ffi`](https://doc.rust-lang.org/std/ffi/index.html)
+
 ## `tests/ui/ffi-attrs/`: `#![feature(ffi_const, ffi_pure)]`
 
 The `#[ffi_const]` and `#[ffi_pure]` attributes applies clang's `const` and `pure` attributes to foreign functions declarations, respectively. These attributes are the core element of the tests in this category.