Added and updated README.rst

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

tests/no_comb/README.rst

diff --git a/tests/no_comb/README.rst b/tests/no_comb/README.rst
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+Forced input to be non-combinational
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+
+In some cases `vlog_to_model` incorrectly detects sequential signals as combinational (such as a FF with enable signal).
+
+A `NO_COMB` attribute is provided to specifically force an input to be non-combinational.