Add some documentation for dsptools et al

docs/Tools/Dsptools.rst

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+Dsptools
+===============================
+
+`Dsptools <https://github.com/ucb-bar/dsptools/>`__ is a Chisel library for writing custom signal processing hardware.
+Additionally, dsptools is useful for integrating custom signal processing hardware into an SoC (especially a Rocket-based SoC).
+
+Some features:
+
+* Complex type
+* Typeclasses for writing polymorphic hardware generators
+  * For example, write one FIR filter generator that works for real or complex inputs
+* Extensions to Chisel testers for fixed point and floating point types
+* A diplomatic implementation of AXI4-Stream
+* Models for verifying APB, AXI-4, and TileLink interfaces with chisel-testers
+* DSP building blocks