Added the contribution guidelines.

Author: Dmitriy Navrotskyy

contributing.md

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+# 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 an individual pull request for each suggestion.
+- Use the following format: `[Bookmark Title](link): Description.`
+    - The title usually consists of several words. For bookmarks of an article or tutorial, use the title of the article.
+    - The description consists of one-two sentences about the bookmark. It is required.
+- Titles should be [capitalized](http://grammar.yourdictionary.com/capitalization/rules-for-capitalization-in-titles.html).
+- 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!