Create CONTRIBUTING.md

CONTRIBUTING.md

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+### Want to contribute? 
+Pull requests and issues are always welcome!
+
+#### How to contribute?
+<ol>
+  <li> Fork the <a href="https://github.com/fayaz07/progress_dialog">repository</a></li>
+  <li> Clone it to your local machine </li>
+  <li> Open the project in your favourite editor </li>  
+  <li> Open cmd/terminal and run <b>flutter clean</b> and then <b>flutter packages get</b> </li>
+  <li> Make the changes </li>
+  <li> Create a <b>Pull Request</b> </li>
+</ol>
+
+#### View the issues [here](https://github.com/fayaz07/progress_dialog/issues)
+
+#### This library is only tested for Android, iOS contributors are most welcome
+
+---