Update the "how to contribute" document.

Also remove an unwanted dep to Markdown and add a dev_requirement.txt file to use with pip.

docs/contribute.rst

@@ -1,13 +1,39 @@
 How to contribute ?
-===================
+###################
+There are many ways to contribute to pelican. You can enhance the
+documentation, add missing features, fix bugs or just report them.
 
-There are features missing from pelican and you now exactly how to implement
-them? That's great! Don't hesitate to fork and make a pull request on github.
+Don't hesitate to fork and make a pull request on github.
+
+Set up the development environment
+==================================
+
+You're free to setup up the environment in any way you like. Here is a way
+using virtualenv and virtualenvwrapper. If you don't have them, you can install
+them using::
+
+    $ pip install virtualenvwrapper
+
+Virtual environments allow you to work on an installation of python which is
+not the one installed on your system. Especially, it will install the different
+projects under a different location.
+
+To create the virtualenv environment, you have to do::
+
+    $ mkvirtualenv pelican --no-site-package
+
+Then you would have to install all the dependencies::
+
+    $ pip install -r dev_requirements.txt
 
 Running the test suite
-----------------------
+======================
 
-It would even be better if you could add tests for this feature. The tests 
-are located under "pelican/tests" and you will need unittests2 to run them::
+Each time you add a feature, there are two things to do regarding tests:
+checking that the tests run in a right way, and be sure that you add tests for
+the feature you are working on or the bug you're fixing.
+
+The tests leaves under "pelican/tests" and you can run them using the
+"discover" feature of unittest2::
 
     python -m unittest2 discover