added a new learning resource called fullstackopen

[Deepesh2023]

A good free resource to learn full stack web development (MERN) provided by the University of Helsinki

Description

Added the new resource to the additional resources section

Motivation and Context

I really think its a good source to learn web dev asap if you have some coding experience. I have, at the time of this pull request completed 4 parts and I can definitly recommend.

How Has This Been Tested?

Tested if the new title and link appears on the readme page on my repo

Types of changes

• [ ] Content Update (change which fixes an issue or updates an already existing submission)
• [x] New Article (change which adds functionality)
• [ ] Documentation change

Checklist:

• [x] My code follows the code style of this project.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have made checks to ensure URLs and other resources are valid