Add CONTRIBUTING.md

Author: donnemartin

CONTRIBUTING.md

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+Contributing
+============
+
+Contributions are welcome!
+
+**Please carefully read this page to make the code review process go as smoothly as possible and to maximize the likelihood of your contribution being merged.**
+
+## Bug Reports
+
+For bug reports or requests [submit an issue](https://github.com/donnemartin/system-design-primer/issues).
+
+## Pull Requests
+
+The preferred way to contribute is to fork the
+[main repository](https://github.com/donnemartin/system-design-primer) on GitHub.
+
+1. Fork the [main repository](https://github.com/donnemartin/system-design-primer).  Click on the 'Fork' button near the top of the page.  This creates a copy of the code under your account on the GitHub server.
+
+2. Clone this copy to your local disk:
+
+        $ git clone [email protected]:YourLogin/system-design-primer.git
+        $ cd system-design-primer
+
+3. Create a branch to hold your changes and start making changes. Don't work in the `master` branch!
+
+        $ git checkout -b my-feature
+
+4. Work on this copy on your computer using Git to do the version control. When you're done editing, run the following to record your changes in Git:
+
+        $ git add modified_files
+        $ git commit
+
+5. Push your changes to GitHub with:
+
+        $ git push -u origin my-feature
+
+6. Finally, go to the web page of your fork of the `system-design-primer` repo and click 'Pull Request' to send your changes for review.
+
+### GitHub Pull Requests Docs
+
+If you are not familiar with pull requests, review the [pull request docs](https://help.github.com/articles/using-pull-requests/).