Create CONTRIBUTING.md

Author: thiennn

CONTRIBUTING.md

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+# Contributing to SimplCommerce
+
+First of all, thanks for taking the time to contribute! :tada::+1:
+
+There are lots of ways to contribute to SimplCommerce and we appreciate the help from the community. 
+You can provide feedback, report bugs, give suggestions, contribute code, help us write documents and participate in the discussion.
+
+# Tips when submit pull requests
+
+- Pull requests should be small and focus. This help us review your pull requests easier and faster.
+- Your code should look consistent with the existing code of SimplCommerce: whitespace, indent, blank lines, curly brackets etc.
+If you have any sugession on the coding style, please create an issue and we can discuss on that one by one.
+