Add an issue template link and a file for info about content suggestions

.github/ISSUE_TEMPLATE/config.yml

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+contact_links:
+  - name: Content Suggestion
+    about: Propose new blocks, items, biomes and other content
+    url: https://github.com/PixelGuys/Cubyz/blob/master/docs/CONTENT_SUGGESTIONS.md

docs/CONTENT_SUGGESTIONS.md

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+## We do not accept low-effort content suggestions
+
+It is easy to suggest something, but often it's much harder to implement and check if it fits the game.
+So, before making a suggestion here on github (you can of course freely discuss ideas on the community discord server), please do the following steps:
+
+- check if it follows the [Game Design Principles](https://github.com/PixelGuys/Cubyz/blob/master/GAME_DESIGN_PRINCIPLES.md)
+- make a reference implementation in the form of an addon, mod or fork of Cubyz (no content from other games)
+- create a pull request with the suggested changes or make an issue using the blank issue template, don't forget to add some screenshots