add contribution guidelines

contributing.md

@@ -0,0 +1,16 @@
+# Contribution Guidelines
+
+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 your list is useful before submitting. That implies it having enough content and every item a good succinct description.
+- A link back to this list from yours, so users can discover more lists, would be appreciated.
+- Make an individual pull request for each suggestion.
+- Titles should be [capitalized](http://grammar.yourdictionary.com/capitalization/rules-for-capitalization-in-titles.html).
+- Use the following format: `[List Name](link)`
+- 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!