Add CONTRIBUTING.md file.

CONTRIBUTING.md

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+# Contributing to Taiga
+
+As an open source project, *taiga* welcomes contributions of many forms.
+
+Examples of contributions include:
+
+- Code patches.
+- Documentation improvements.
+- Bug reports.
+- Patch reviews.
+
+Before start developing one big feature (with intentions of including it on taiga code base), it is
+strongly recommended chat about it using our [mailing list](http://groups.google.com/d/forum/taigaio).
+
+## License ##
+
+Every code patch accepted in taiga codebase it is licensed under AGPL v3.0. You should care
+about it and not include code that can not be licensed under that license.
+You can see the complete license on `LICENSE` file on the root of repository.