Added contributing.md

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+contributing.md export-ignore
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contributing.md

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+# How to contribute
+
+There are several ways to help out:
+
+* Create an issue on GitHub, if you have found a bug
+* Write test cases for open bug issues
+* Write fixes for open bug/feature issues, preferably with test cases included
+* Contribute to the [documentation](https://github.com/LeanMapper/LeanMapper.github.io) or [examples](https://github.com/LeanMapper/examples)
+
+
+## Issues
+
+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.
+
+
+## Contributing
+
+### Preparing environment
+
+Start with [forking](https://help.github.com/articles/fork-a-repo) [Lean Mapper on GitHub](https://github.com/Tharos/LeanMapper).
+Carefully [set up](https://help.github.com/articles/set-up-git) your local Git environment, configure your username and email, these credentials will identify your changes in Lean Mapper history.
+
+
+### Working on your patch
+
+Before you start working on your patch, create a new branch for your changes.
+
+```
+git checkout -b new_branch_name
+```
+
+
+### Testing your changes
+
+You need to install [Nette Tester](https://tester.nette.org/). The easiest way is to call `composer install` in repository root.
+
+Now you should be able to run tests. On unix-like systems run following command in your terminal:
+
+```
+vendor/bin/tester -p php -c tests/php-unix.ini
+```
+
+**Note:** the tests require these PHP extensions:
+
+* `json`
+* `sqlite3`
+* `tokenizer`
+
+----
+
+Please do not fix whitespace, format code, or make a purely cosmetic patch.
+
+Thanks!