Add CONTRIBUTING.md file

Fixes #3

Author: xredo

CONTRIBUTING.md

@@ -0,0 +1,19 @@
+# Contributing
+
+This project is mainly targeted for MarsBased developers 👽 but we love pull request from earthlings too 🌍.
+
+Before contributing make sure that there is not any [issue](https://github.com/MarsBased/pathfinder/issues) already created that covers the same problem / feature that you want to add.
+
+We recommend you to create an issue before starting the development to better organize contributions.
+
+## How to
+
+First fork the repository on your own account and clone it.
+
+```git clone git@github.com:your-username/pathfinder.git```
+
+Make your feature addition or bug fix in a featurebranch.
+
+Push your feature branch to GitHub and send us a [Pull Request](https://github.com/marsbased/pathfinder/compare)
+
+Please, include a description of your changes and reference the issue that you are fixing in your commit.