Create contributing.md

Author: aj07

contributing.md

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+If you want to contribute to this list (please do), send me a pull request. Since we want this list to be useful in the long run, **please submit high quality links only**.
+
+## Adding to this list
+
+Please ensure your pull request adheres to the following guidelines:
+
+- **Please make an individual pull request for each suggestion.**
+- Please, don't directly send PR to master. Make branch, commit your changes and then send PR.
+- The pull request and commit should have a useful title.
+- Please search previous suggestions before making a new one, as yours may be a duplicate.
+- Make sure your link has a useful and relevant title.
+- Please use [title-casing](http://titlecapitalization.com) (AP style). 
+- Please use the following format: `[Useful Title](link)`
+- Link additions should be added to the bottom of the relevant category.
+- New categories or improvements to the existing categorization are welcome.
+- Please check your spelling and grammar.
+
+Thank you for your suggestions! Happy Coding!!