executable tutorial: nixpgs for reproducible environments (#2455)

* feat: added executable tutorial proposal

* fix: corrected some spelling mistakes

contributions/executable-tutorial/arberl-mateusma/README.md

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+# Assignment Proposal
+
+## Title
+
+Nixpkgs and its use for reproducible environments for better local testing.
+
+## Names and KTH ID
+
+- Arber Limani ([email protected])
+- Mateus Marinheiro ([email protected])
+
+## Deadline
+
+- Task 3
+
+## Category
+
+- Executable Tutorial
+
+## Description
+
+We will give a brief overview of the Nixpkgs syntax as well as its benefits while teaching how to apply it to real world scenarios. We will also provide a guide on how to change your environments to test your code locally before committing it to your repo.
+
+**Relevance**
+
+It is very important to test your code before you submit it, even if there are CI actions in place. As a team starts to grow, different developers with different machines with different environment states. It is therefore essential to ensure that even if machines have different configurations the result of testing should be the same.