Add Contributing documents

Author: guillaumechereau

CONTRIBUTING.md

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+# Contributing to Goxel
+
+
+## CLA
+
+For the moment I ask every contributors to sign a CLA (contributor license
+agreement) document that gives me the right to relicense the code under any
+other license.
+
+To sign the CLA you can either get the pdf and send me the signed version
+by email, or contact me and I'll send you a link to a form that allows to sign
+it online.
+
+Link to the pdf:
+
+- For individuals:
+    https://guillaumechereau.github.io/goxel/cla/goxel-cla-individual.pdf
+
+- For company:
+    https://guillaumechereau.github.io/goxel/cla/goxel-cla-entity.pdf
+
+You can send the signed documents to [email protected].