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.

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• The list I added is a non-generated Markdown file in a GitHub repo,
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