docs(readme): add contribution setting image

Author: bcanseco

.github/images/private-contributions.png

@@ -40,6 +40,8 @@
    ![](./.github/images/name-new-file.png)
 1. Paste in one of the YAML file contents below, depending on what you want to do. Be sure to update `GIT_EMAIL`.
 1. Click the **Commit new file** button at the bottom of the page. You're all set!
+   * Note that you must enable the option below in your contribution settings for private commits to count on the graph.
+   ![](.github/images/private-contributions.png)
 
 If you change your mind about these commits later, you can delete the repository and they'll disappear from your contribution graph.