doc: add tests

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

diff --git a/doc/contributions/tests.md b/doc/contributions/tests.md
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+# Tests
+
+Currently, shadow only provides system tests.
+
+## System tests
+
+These type of tests are written in shell. Unfortunately, the testing framework
+is tightly coupled to the Ubuntu distribution and it can only be run in this
+distribution. Besides, if anything fails during the execution the system can
+be left in an unstable state. Taking that into account you shouldn't run this
+workflow in your host machine, we recommend to use a disposable system like a
+VM or a container instead.
+
+You can execute system tests by running:
+
+```
+cd tests && ./run_all`.
+```