doc: Add a good resource for learning to write Rust FFI.

doc/HACKING/GettingStartedRust.md

@@ -43,6 +43,9 @@ For learning how to write unsafe Rust, read
 For learning everything you ever wanted to know about Rust macros, there is
 [The Little Book of Rust Macros](https://danielkeep.github.io/tlborm/book/index.html).
 
+For learning more about FFI and Rust, see Jake Goulding's
+[Rust FFI Omnibus](http://jakegoulding.com/rust-ffi-omnibus/).
+
  Compiling Tor with Rust enabled
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