Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Contribution Guidelines
+> The pull request should have a useful title. Pull requests with `Update readme.md` as title will be closed right away. Please make sure you checked every box in the PR template.
+
+## Adding an item to the list
+1. Make sure you are editing README.md only, no other files are allowed to be edited.
+2. Add your item to the category it belongs to. Try to avoid creating completely new categories.
+3. Make sure your item is added to the bottom of the items listed in a category, don't add it in at the middle.
+4. Don't submit dead links and try to always submit HTTPS links.