Add Slim

[nekofar]

https://github.com/nekofar/awesome-slim

The list is about Slim, a PHP micro framework that helps you quickly write simple yet powerful web applications and APIs. It should be included since (1) the isn't already a Slim awesome list and (2) Slim is a popular framework for PHP back-end development.

By submitting this pull request I confirm I've read and complied with the below requirements.

Failure to properly do so will just result in the pull request being closed and everyone's time wasted. Please read it twice. Most people miss many things.

• [x] I have read and understood the contribution guidelines.
• [x] This pull request has a descriptive title. For example, Add [Name of List], not Update readme.md or Add an awesome list.
• [x] The list I added has been around for at least 20 days,
• [x] The list I added is a non-generated Markdown file in a GitHub repo,
• [x] The list I added is not a duplicate