added contributing file

Author: cayetanobv

CONTRIBUTING.md

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+# How to contribute
+
+We are really glad you are reading this, because we need volunteer developers to help this project.
+
+If you have not already, come find us in [![Join the chat at https://gitter.im/pgRouting/pgrouting](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/pgRouting/pgrouting?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge) or contact us via [mailing list](http://lists.osgeo.org/mailman/listinfo/pgrouting-dev). We want you working on things you're excited about. Drop a message and if some one can assist you, will contact you back ASAP.
+
+Here are some important resources:
+
+  * [pgRouting Documentation](http://docs.pgrouting.org/) The plugin is based on pgRouting
+  * [Wish list on the wiki](https://github.com/pgRouting/pgrouting/wiki/GSoC-Ideas) is the foot view of what we think the community needs and can be done by GSoC students.
+  * [Additional wish list](https://github.com/pgRouting/osm2pgrouting/issues?q=is%3Aopen+is%3Aissue+label%3A%22Functionality+Request%22).
+  * Participate on the [discussions](https://github.com/pgRouting/osm2pgrouting/issues?q=is%3Aopen+is%3Aissue+label%3ADiscussion).
+  * Participate fixing [Documentation](https://github.com/pgRouting/osm2pgrouting/issues?q=is%3Aopen+is%3Aissue+label%3ADocumentation).
+  * Find a bug? [Let us know](https://github.com/pgRouting/osm2pgrouting/issues).
+
+## Contribution agreement
+
+Any kind of contribution will automatically fall to the following Licences:
+
+- Code contribution: GNU GENERAL PUBLIC LICENSE Version 2,
+  - Directly by making a pul explicit pull request.
+  - Indirectly by posting code on issues/wiki/gitter/mailng lists
+- Documentation contribution:
+  - Creative Commons Attribution-Share Alike 3.0
+
+## Submitting changes
+
+
+Write a clear log message for your commits. One-line messages are fine for small changes, but bigger changes should have more information.
+Fill the commit message template.
+
+## Coding conventions
+
+This is open source software. Consider the people who will read your code, and make it look nice for them.
+
+  * We indent using four spaces (soft tabs)
+
+Thanks,
+pgRouting team