accumulate: move important hint to instructions

exercises/practice/accumulate/.docs/hints.md

@@ -8,7 +8,3 @@ It may help to look at the Fn\* traits:
 Help with passing a closure into a function may be found in
 the ["closures as input parameters" section](https://doc.rust-lang.org/stable/rust-by-example/fn/closures/input_parameters.html) of
 [Rust by Example](https://doc.rust-lang.org/stable/rust-by-example/).
-
-The tests for this exercise will cause compile time errors,
-if your function signature does not fit them, even when they're not run.
-You may want to comment some tests out and generalize your solution piece by piece.

exercises/practice/accumulate/.docs/instructions.append.md

@@ -0,0 +1,6 @@
+# Instructions append
+
+## Workflow
+
+The tests for this exercise will cause compile time errors if your function signature does not fit them, even when they're not run.
+You may want to comment some tests out and generalize your solution piece by piece.