Add some guidline for dev

CONTRIBUTING.md

+We are glad that you are considering contributing to navipy, and are
+looking forward to your contribution.
+
+# Pre-requisit
+We strongly recommand you to download, install, test, and compile the documentation
+
+## Installation
+```
+git clone git@gitlab.ub.uni-bielefeld.de:olivier.bertrand/navipy.git
+cd navipy
+python setup.py install
+```
+
+## Unittesting
+From the root directory of the repository
+
+```
+python -m unittest discover navipy
+```
+
+## Blend-unittesting
+Several functionalty of navipy requires Blender, and thus code has to be run within blender (see doc for details). Unittesting of function requiring blender can be done as follow
+
+```
+blendunittest
+```
+
+Note: this command require blender to be installed
+
+## Doc building
+The doc is contained in doc/source
+
+```
+cd doc
+make html
+```
+
+# Convention
+We follow the PEP8 recommendation and the code should be formatted as such.
+
+# Testing
+Ideally every function should be properly tested. Test file should match the pattern 'test*.py' here the * should match the module to be tested. For example to test function within maths.py, a file named test_maths.py should be created and stored in the same folder than maths.py
+
+Certain functionalities require the use of blender. Such test need to be stored in a file 'blendtest*.py'. Note that the blendunittest command uses the blender environment navipy/resources/twocylinders_world.blend. Therefore your test function should use this environment as well.
+
+# Documentation
+The documentation is based on Sphinx syntax. Although documenting functions is a must, tutorial is a bonus that will strongly encourage.