Update CONTRIBUTING

Author: mo8it

CONTRIBUTING.md

@@ -32,11 +32,14 @@ If you need any help with it or face any Git related problems, don't hesitate to
 It may take time to review your pull request.
 Please be patient 😇
 
+When updating an exercise, check if its solution needs to be updated.
+
 ## Adding An Exercise
 
 - Name the file `exercises/yourTopic/yourTopicN.rs`.
-- Make sure to put in some helpful links, and link to sections of the book in `exercises/yourTopic/README.md`.
-- Add a (possible) solution at `solutions/yourTopic/yourTopicN.rs` with comments and links explaining it.
+- Make sure to put in some helpful links, and link to sections of The Book in `exercises/yourTopic/README.md`.
+- In the exercise, add a `// TODO: …` comment where user changes are required.
+- Add a solution at `solutions/yourTopic/yourTopicN.rs` with comments explaining it.
 - Add the [metadata for your exercise](#exercise-metadata) in the `rustlings-macros/info.toml` file.
 - Make sure your exercise runs with `rustlings run yourTopicN`.
 - [Open a pull request](#pull-requests).
@@ -49,7 +52,9 @@ The exercise metadata should contain the following:
 [[exercises]]
 name = "yourTopicN"
 dir = "yourTopic"
-hint = """A useful (multi-line) hint for your exercise."""
+hint = """
+A useful (multi-line) hint for your exercise.
+Include links to a section in The Book or a documentation page."""
 ```
 
 If your exercise doesn't contain any test, add `test = false` to the exercise metadata.