Add contribution guide, closes #7

CONTRIBUTING.md

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+Feel free to hack Gaphor. Patches are welcome.
+
+Fetching Development Dependencies
+=================================
+
+Gaphor uses Easy Install to manage in easy way its dependencies.
+
+To fetch and install dependencies as non-root user
+
+1. Create ~/.pydistutils.cfg file
+
+    [install]
+    install_lib = ~/opt/share/python2.5/site-packages
+    install_scripts = ~/opt/bin
+
+2. Create (or extend) PYTHONPATH variable
+
+    export PYTHONPATH=~/opt/share/python2.5/site-packages
+
+3. Run setup.py script to fetch and install dependencies
+
+    python setup.py develop
+
+Prefix ~/opt can be changed to something more suitable for your setup.
+
+
+Above is based on
+
+    http://peak.telecommunity.com/DevCenter/EasyInstall#traditional-pythonpath-based-installation
+
+
+Running Tests
+=============
+To run tests on Unix machine
+
+    Xvfb :2.0 &
+    DISPLAY=:2.0 nosetests gaphor/ 2>&1 | tee tests.log
+
+
+Structure
+=========
+
+Gaphor contains the following modules:
+
+UML
+---
+The UML module contains the UML 2.0 data model. This part is
+quite stable and it is unlikely that code has to be changed
+here.
+
+  NOTE:	The code is generated from a Gaphor model: uml2.gaphor. This
+	file can be loaded in gaphor.
+
+diagram
+-------
+The diagram module contains items that can be placed in diagrams.
+In most cases the classes NamedItem and Relationship can serve
+as bases for your class.
+
+ui
+--
+The user interface. This is where most of the work is to be done.
+
+misc
+----
+Some utility stuff, such as Actions and aspects are put in here.