Add README to tests.

Author: ayjayt

src/py/tests/README.md

@@ -0,0 +1,11 @@
+- test_public_api.py does render tests of renderers in __init__.py
+  - it does not parametrize
+- test_init.py tests that wrappers are proper and pass args/kwargs
+  - it does parameterize
+- test_kaleido tests the substantial parts of kaleido and its returns
+  - it has incomplete parameterizing
+
+
+
+Parameterizing actual renders would be a huge burden, but integration tests
+with mocks test it more completely