Additional info for adding exercises in README

Add an example exercise entry for config.json in the README to make it easier for new contributors to find info in one place.

README.md

@@ -12,6 +12,8 @@ Please see the [contributing guide](https://github.com/exercism/x-common/blob/ma
 
 ## Working on the Exercises
 
+### Implementing an exercise
+
 A pool of exercises can be found in the [x-Common repo](https://github.com/exercism/x-common).
 
 Exercises for the Julia track go in the `exercises` directory and should follow the following filename conventions:
@@ -26,5 +28,19 @@ Replace `<slug>` with the exercise slug of the exercise you're working on.
 
 See [Issue #2](https://github.com/exercism/xjulia/issues/2) for discussion on the structure.
 
+### Adding it to config
+
+Make sure to add the exercise to the `config.json` file, by adding an entry to the `exercises` array:
+```json
+"exercises": [
+  {
+    "slug": "hello-world" ,
+    "difficulty": 1,
+    "topics": ["strings"]
+  }
+]
+```
+If possible, add info on which topics the exercise is about and estimate a difficulty level from 1 to 10. We can adjust these later on when we know more about the exercise and how users solve them.
+
 ### Testing the example solutions
 Test your example solutions by running `julia runtests.jl` in the project directory. Specify exercise slugs as arguments to run only certain exercises: `julia runtests.jl <slug>`.