Add note on thunks (#402)

* Add note on thunks

* Update docs/src/FAQ.md

Co-authored-by: Lyndon White <[email protected]>

Co-authored-by: Lyndon White <[email protected]>

Author: willtebbutt

docs/src/FAQ.md

@@ -97,3 +97,13 @@ The list here is incomplete, but is vetted for quality.
 
  - ["Evaluating Derivatives"](https://dl.acm.org/doi/book/10.5555/1455489) (by Griewank and Walther) is the best book at least for reverse-mode.
 It also covers forward-mode though (by its own admission) not as well, it never mentioned dual numbers which is an unfortunate lack.
+
+## Is removing a thunk a breaking change?
+Removing thunks is not considered a breaking change.
+This is because (in principle) removing them changes the implementation of the values
+returned by an rrule, not the value that they represent.
+This is morally the same as similar issues [discussed in ColPrac](https://github.com/SciML/ColPrac#changes-that-are-not-considered-breaking), such as details of floating point arithmetic changing.
+
+On a practical level, it's important that this is the case because thunks a bit of a hack,
+and over time it is hoped that the need for them will reduce, as they increase
+code-complexity and place additional stress on the compiler.