Create CONTRIBUTING.md

Author: castarco

CONTRIBUTING.md

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+BigNumbers Contributing Guidelines
+====================================
+
+Contributing with Code
+----------------------
+
+In first place, if you want to contribute code to php-bignumbers, you should have installed some PHP version,
+preferably recent versions (PHP>=5.3, but recommended PHP>=5.6), or *HHVM*.
+
+If you don't know where to start, you can try searching inside the
+[issues list](https://github.com/Litipk/php-bignumbers/issues). Specially the ones marked with the
+[help wanted](https://github.com/Litipk/php-bignumbers/labels/help%20wanted) tag.
+
+Some extra advices related on good practices:
+ * We try to follow the PSR standards, in special **PSR-1**, **PSR-2** and **PSR-4**.
+ * Every new added feature should come with unit tests, stability and code correctness is an
+   important priority of this project.
+
+
+Contributing with Documentation
+-------------------------------
+
+If you want to contribute documentation, you're welcome! :) , BUT, actually the documentation is pretty bad,
+and it's perfectly undersandable that this task isn't very attractive.
+
+Contributing with Bug Reports
+-----------------------------
+
+A very important part of every software project is the bugs tracking and solving process. You can send us bug reports through the [tracker](https://github.com/Litipk/php-bignumbers/issues).
+
+It's desirable that every bug report explains the bug in a way that the developers can easily reproduce it. The following points are very helpful in order to achieve reproducibility:
+* Example code.
+* php-bignumbers version.
+* PHP version
+
+Contributing with New Ideas
+---------------------------
+
+You can use the github [tracker](https://github.com/Litipk/php-bignumbers/issues) to send us new ideas, even if those ideas aren't bug reports. In such case, adding something like "SUGGESTION: ...", "IMPROVEMENT: ..." to the report bug it's a good idea.
+
+You can also discuss your ideas with us in our [Gitter chat room](https://gitter.im/Litipk/php-bignumbers).