Add new illustration links to README

[Abrish-21]

This pull request updates the README file to include two new illustration resources:

1. Open Peeps: A customizable, hand-drawn illustration library by Pablo Stanley, free for commercial and personal use under the CC0 License.
2. Overflow Design: Offers a variety of modern design illustrations and icons, useful for web and app design projects. These additions enhance the repository's collection of design assets. They are also free to use.

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[markodenic]

@Abrish-21 We already have a lot of illustration resources. Please, consider contributing to some other category with fewer resources.