add Elixir project

[bxdoan]

Description

Motivation and Context

I was going to add a good Elixir tutorial that uses Mnesia and Phoenix

How Has This Been Tested?

I checked the links to make sure they linked to the proper pages

Types of changes

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

Checklist:

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