Add a README.rst for sys_path_hacks

sys_path_hacks/README.rst

+This is a transitional folder as we move away from modifying ``sys.path`` in django config.
+
+See `the ADR`_ for more details.
+
+.. _the ADR: https://github.com/edx/edx-platform/blob/master/docs/decisions/0007-sys-path-modification-removal.rst