Add CONTRIBUTING

CONTRIBUTING.md

+# Contributing
+
+We use poetry to build and distribute our package. Make sure you install poetry. Then clone the repository with
+
+    git clone https://lab.compute.dtu.dk/imagelab/photometric_stereo.git
+
+Use
+
+    poetry install
+
+to install the virtual envronment with packages, and
+
+    poetry run python -m photometric_stereo
+
+to test the application.
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