Document branch naming convention for PRs

Author: spawnia

CONTRIBUTING.md

@@ -11,7 +11,7 @@ For this project specifically, follow these steps:
 1. Fork the project
 2. Clone the repository
 3. [Set up the project](#setup)
-4. Create a branch
+4. Create a branch without a category prefix (for example, `fix-foo`, not `fix/foo`)
 5. Code according to the [guidelines](#code-guidelines) and [style](#code-style)
 6. [Test your changes](#testing)
 7. Commit and push