Add quick contribution guideline.

Author: darkriszty

CONTRIBUTING.md

@@ -0,0 +1,13 @@
+# Contributing to Markdown Table Prettifier
+
+## How Can I Contribute?
+
+### Reporting Issues (bugs, imrpovement ideas, etc.)
+
+One way to contribute is through the issues page. Before creating a new issue, please check the current list as you might find out that you don't need to create one. Please include as many details as possible in your issue.
+
+### Contributing code
+
+Code contributions must be linked to an issue. Before starting to write code for it, please check if the issue is not already taken.
+
+All code must be integrated via a Pull Request, where each Pull Request must solve one issue.