Updated CONTRIBUTING.md (#9117)

After having to turn down #9110 I read our "How to Contribute" documentation.
We mention that a new branch should be created, however we may not be explicit enough.
Added a note to resolve that.

.github/CONTRIBUTING.md

@@ -65,7 +65,7 @@ Suggesting Enhancements
 
 1. Make sure you have a [GitHub account](https://github.com/signup/free).
 2. Next, you will need to [fork rAthena](https://help.github.com/articles/fork-a-repo/#fork-an-example-repository) to your account.
-3. Before making changes, make sure you [create a new branch](https://help.github.com/articles/creating-and-deleting-branches-within-your-repository/) for your working tree.
+3. Before making changes, make sure you [create a new branch](https://help.github.com/articles/creating-and-deleting-branches-within-your-repository/) for your working tree. Never use your **master** branch! :bangbang:
 4. After completing your changes, commit and push it to your branch.
 5. Now you are ready to [create a Pull Request](https://help.github.com/articles/creating-a-pull-request/) for rAthena!
   * Upon creating the Pull Request, make sure you follow our [template](https://github.com/rathena/rathena/blob/master/.github/PULL_REQUEST_TEMPLATE.md) and provide the required information.