Add contibuting.md

Author: vkbansal

CONTRIBUTING.md

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+# Contributing
+First off, thanks for taking the time to contribute.
+
+Please read https://reactjs.org/ and the Code of Conduct before opening an issue.
+
+### Found a bug?
+
+##### Before Submitting A Bug Report
+- Please read the [API documentation](https://github.com/vkbansal/react-contextmenu#api) thoroughly
+- Perform a [cursory search](https://github.com/vkbansal/react-contextmenu/issues?utf8=%E2%9C%93&q=is%3Aissue) to see if the problem has already been reported. If it has, add a comment to the existing issue instead of opening a new one.
+
+##### Submiting a (good) bug report
+
+-   **Use a clear and descriptive title** for the issue to identify the problem.
+-   **Provide specific examples to demonstrate the steps**. Include links to CodePen/JS Bin or GitHub projects, or copy/paste-able snippets, which you use in those examples. If you're providing snippets in the issue, use [Markdown code blocks](https://help.github.com/articles/markdown-basics/#multiple-lines).
+-   **Describe the behavior you observed after following the steps** and point out what exactly is the problem with that behavior.
+-   **Explain which behavior you expected to see instead and why.**
+-   **Include screenshots and animated GIFs** which show you following the described steps and clearly demonstrate the problem.
+
+### Proposing New or Changed API?
+
+Please provide thoughtful comments and some sample code. Proposals without substance will be closed.
+
+### Making a Pull Request
+
+Pull requests need only the 👍 of admin or two or more collaborators to be merged.
+
+##### Linting and Tests
+
+-   All the commits must pass lint and tests.
+-   All commits that fix bugs or add features need a test.
+
+You can run lint + tests  via:
+
+```
+npm test
+```
+
+If you just want to run lint:
+
+```
+npm run lint
+```
+
+If you just want to run all tests:
+
+```
+npm run test:only
+```
+
+##### Development
+
+You can start webpack & dev server that watches for changes and build the examples via:
+
+```
+npm start
+```
+
+
+