added contributing.md

inspiration https://github.com/necolas/issue-guidelines

contributing.md

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+How to contribute & use the issue tracker
+=========================================
+
+The issue tracker is the preferred channel for bug reports, features requests
+and submitting pull requests, but please respect the following restrictions:
+
+* Please **do not** use the issue tracker for personal support requests (use
+  [Nette forum](http://forum.nette.org) or [Stack Overflow](http://stackoverflow.com)).
+
+* Please **do not** derail or troll issues. Keep the discussion on topic and
+  respect the opinions of others.
+
+* Use the GitHub **issue search** — check if the issue has already been
+   reported.
+
+A good **bug report** shouldn't leave others needing to chase you up for more
+information. Please try to be as detailed as possible in your report.
+
+**Feature requests** are welcome. But take a moment to find out whether your idea
+fits with the scope and aims of the project. It's up to *you* to make a strong
+case to convince the project's developers of the merits of this feature.
+
+Nette welcomes **pull requests**. If you'd like to contribute, please take a moment
+to [read the guidelines](http://nette.org/en/contributing) in order to make
+the contribution process easy and effective for everyone involved.
+
+Thanks!