Create CONTRIBUTING.md

Gathros pointed out, that we should add a CONTRIBUTING.md.
This PR adds one. It includes links to the How to Contribute wiki page and to the video series by Buttercak3.

Author: june128

CONTRIBUTING.md

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+# Contributing
+
+A contribution guide on how to contribute to the Arcane Algorithm Archive (AAA) can be found on this Wiki page:
+https://github.com/algorithm-archivists/algorithm-archive/wiki/How-to-Contribute
+
+The community member Buttercak3 also created a video series, explaining the contribution process of the AAA.
+You can find a playlist with all videos here:
+https://www.youtube.com/playlist?list=PL5NSPcN6fRq2vwgdb9noJacF945CeBk8x