Add some documentation for dsptools et al

docs/Tools/Chisel-Testers.rst

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+Chisel Testers
+==============================
+
+`Chisel testers <https://github.com/freechipsproject/chisel-testers>`__ is a library for writing tests for Chisel designs.
+It provides a Scala API for interacting with a DUT.
+It can use multiple backends, including :ref:`Treadle` and Verilator.