Add contribution guideline

contributing.md

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+# Adding to this list
+
+ Please ensure your pull request adheres to the following guidelines:
+
+ - Search previous suggestions before making a new one, as yours may be a duplicate.
+ - Make sure the list is useful before submitting. That implies it has enough content and every item has a good succinct description.
+ - Make an individual pull request for each suggestion.
+ - Use the following format: `[List Name](link)`
+ - Link additions should be added to the bottom of the relevant category.
+ - New categories or improvements to the existing categorization are welcome.
+ - Check your spelling and grammar.
+ - Make sure your text editor is set to remove trailing whitespace.
+ - The pull request and commit should have a useful title.
+
+ Thank you for your suggestions!