add initial directory for batch testing

tests/batch/readme.rst

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+
+***********
+Batch Tests
+***********
+
+These tests aren't meant to be executed in an isolated manner (as with unit-tests).
+
+They are meant to run against real blender projects, checking the output is valid.
+
+So for example we can point a batch test at a large repository containing ``.blend`` files
+and check that we can manipulate the files, keeping them valid.
+