✨ Add formatting guidelines to contributing.md

Author: sreekaransrinath

CONTRIBUTING.md

@@ -19,6 +19,8 @@
 
         Example: `:bug: fix: Resolve 'isCorrect' function error (#0)`
 
+    -   Format your code with [Prettier](https://prettier.io/), and set "printWidth" to 80.
+
 -   Open a Pull Request (PR).
     -   [Learn how to open a PR](https://github.com/firstcontributions/first-contributions).
     -   Solve one issue per PR, **without any extra changes**.