Make file test readme refer to toolchain docs (#5105)

Instead of duplicating the content (slightly differently)

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

testing/file_test/README.md

@@ -203,9 +203,8 @@ Supported comment markers are:
     path to the test's arguments.
 
     This can be used to provide a minimal `Core` package (and `prelude` library)
-    in toolchain tests by providing a path to a file with
-    `package Core library "prelude";` as its first line and adding
-    `--custom-core` to `EXTRA-ARGS`.
+    in toolchain tests. See the
+    [toolchain docs](/toolchain/docs/adding_features.md) for how.
 
 -   ```
     // SET-CAPTURE-CONSOLE-OUTPUT