doc: add license

Signed-off-by: Iker Pedrosa <ipedrosa@redhat.com>

doc/contributions/license.md

@@ -0,0 +1,10 @@
+# License
+
+All new source code committed to the shadow project is assumed to be made
+available under the [BSD-3-Clause](../../COPYING) license unless the submitter
+specifies another license at that time. The shadow maintainers reserve the
+right to refuse a submission if the license is deemed incompatible with the
+goals of the project.
+
+**Note**: old code may be made available under another license, check the
+license tag for each file to get additional information.