Added README.md to the test.

Signed-off-by: Maciej Kurc <mkurc@antmicro.com>

tests/internal_conn/README.md

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+# A test for cells with "passthrough" modes and direct pin-to-pin connections.
+
+This tests checks two cases:
+
+1. A cell may have some of its input pins connected directly to its output pins internally. In such a case that connection has to be expressed within its interconnect. This test does exactly that.
+2. Given a cell which has a "passthrough" mode with no child cells and a direct input to output connection, no pb_type for the "passthrough" mode should be generated. Moreover, the interconnect that holds the input to output connection should be defined directly undet the "mode" tag.
+